Integrating APRS on Voice Channels


Copyright 1993,4,5,6 WB4APR

NEW: See changes made in production units beginning at Dayton (May97) Only APRS791 and later will properly decode the production Mic-E.

The APRS Mic-Encoder eliminates the need for every mobile to have a TNC, digital radio, and second antenna by simply integrating the position report into a very brief tone burst at the end of a voice transmission over any two-way radio. With the Mic-E, no additional hardware is needed in the vehicle, other than a GPS unit. The system not only reports position, course and speed, and vehicle type, but also one of 7 messages, a BeaconText, and 3 analog telemetry values!

By transmitting a position report in a 0.3 sec packet burst at the end of a voice transmission, not only is this a period of dead time due to the almost universal courtesy beeps found on amateur repeaters, but the tone burst can be easily muted out at the repeater receiver, so that the other mobile users do not hear it. In this way it will be virtually transparent to voice repeater operation. The APRS Mic-E achieves the complete position report, course, speed, and digipeater information in about 30 bytes including header, instead of the 90 or more bytes in a normal APRS position report.

At the voice repeater receiver, a TNC picks off the position report and digipeats it out onto the dedicated APRS digital frequency for mobile position reporting. Although any TNC can be used, a special TNC is being developed that determines the path based only on the bits in the TO-SSID instead of the usual long string of digipeaters. This makes the packet very short. In addition, the special APRS NODE TNC appends the repeater frequency onto the end of the position report so that digital users can see where the packet originated. If all voice repeaters digipeated onto the same digital position reporting channel (usually 145.79 if available) then anyone monitoring the APRS frequency will see ALL mobile position reports from ALL GPS mobiles on ALL frequencies!

APRS MIC ENCODER: The Mic-E met many design objectives:

PACKAGING: Although the electronics could be miniaturized into about 1 cubic inch, the requirement for user access to switches and the requirement for withstanding the pulling and tugging on the MIC cord results in a robust dash board box design. The cicruit may be powered by the mic connector and the only external input is via a standard 1/8 inch phone jack or DE-9 connector to receive the NMEA data from the GPS unit. This makes the entire mobile vehicle position reporting system as portable as the microphone! Simply move the MIC from vehicle to vehicle, and as long as the radios are compatible at the MIC connector, then the vehicles are GPS ready! The front panel for the MIC ENCODER is shown below. It connects between your microphone and radio and has internal jumpers for configuring to any 8 wire microphone using either the standard round connector or an RJ-45.

  |  PERIOD    R C V   OMNI    DIR      SSID     DIGI    AUTO   POWER   |
  |            L V L     7  N  S           7  0  1               O N    |
  |    /^             6         E |     6         2                    |
  |   /        (O)   5   / -    W     5   / -    3     O       O     |
  |  | (O) |         4   |     |   N+  4   |     |   4   (O)     (O)    |
  |   ___/     (O)   3    - /   S+    3    - /   5                   |
  |  min               2         E+      2         6                    |
  |            P T T     1  0  W+          1  0  7                      |
  |            O U T    P  A T  H       M E S S A G E   O F F   O F F   |
On the APRS MIC ENCODER, the configuration switches give the operator real-time control over other dynamic MIC choices as follows:

ON/OFF Used to enable or disable the Mic Encoder packets
AUTO Enables auto-packets if repeater has been quiet for N secs.
PATH Used to set digi HOPS or North/South/East or West routes
MSG Selects 1 of 7 messages AND selects DIGI or SSID mode
PERIOD Used to change the reporting period

Normally the MIC encoder will only send a POSIT if the POSIT timer has elapsed AND the user has been talking AND releases his PTT. In the AUTO mode, however, after a specified AUTO time period, AND after the repeater has been silent for the QUIET period, then a position packet will be initiated and transmitted. Most voice repeaters will never even key up on such a brief burst due to their built in ker-chunk filters. The TNC on the repeater input, however, will hear it and digipeat it normally. Note, for the QUIET timer to work properly, the receiver's audio must be set high enough to occassionally flicker the RCV LED.

MESSAGE BITS: The 3 message bits select one of seven pre-defined messages as follows. In addition, all message numbers above 3 will change your symbol color as shown.

0 normal Off duty
1 normal Enroute
2 normal In Service
3 normal Returning
4 dim yel Committed
5 Brt yel Special
6 dim red PRIORITY Trips alarms & centers all maps to unit
7 Brt red EMERGENCY! Trips alarms & centers all maps to unit


There are actually two routing systems, one that can use standard TNC's at the repeater and the other that takes advantage of a new specialized APRS NODE TNC. We will call the standard mode the DIGI mode and the other, the SSID mode. In the DIGI mode, the path switches just select how many hops along one of two preset digi strings will be transmitted in the packet. The advantage of this method is that it is compatible with ANY TNC and will work with existing systems. The disadvantage is that each digi hop takes 7 bytes, and just a 3 hop path almost doubles the length of the packet. The second mode uses only the 4 SSID bits for all routing information. This keeps the packet short, while also allowing for up to 7 hops in all directions! The high order PATH bit selects between OMNI or DIRECTIONAL routing and the 3 routing bits are used to tell the repeater how to route the packet.

The following table shows how the routing is handled in both the DIGI and SSID mode. In DIGI mode the TO-SSID is always 0 and the actual digi path is included in the packet. In SSID mode, there are no digis transmitted and the switch setting are transmitted in the 4 bits of the TO SSID. In the following example, assume the MIC-ENCODER has been loaded with the digi string of RELAY,WIDE,WIDE,DIG4,DIG5,DIG6,DIG7

0 000 0 none none
0 001 1 RELAY WIDE-1
0 100 4 DIG4 WIDE-4
0 101 5 DIG4,DIG5 WIDE-5
0 110 6 DIG4,DIG5,DIG6 WIDE-6
0 111 7 DIG4,DIG5,DIG6,DIG7 WIDE-7
1 000 8 none NORTH UNPROTO path
1 100 12 DIG4 NORTH UNPROTO path + WIDE
1 101 13 DIG4,DIG5 SOUTH UNPROTO path + WIDE
1 110 14 DIG4,DIG5,DIG6 EAST UNPROTO path + WIDE

In this example, the Mic-Encoder was programmed with a via path of RELAY,WIDE,WIDE,DIG4,DIG5,DIG6,DIG7.

CONVENTIONAL DIGIPEAT ROUTING: First notice that in the DIGI mode, the paths 0 through 3 simply select the number of digi hops in the original string to use. The paths 4 to 7 start over again at the 4th position. This can be thought of as a completely independent second DIGI string. Typically you would set the path to RELAY,WIDE,WIDE,WIDE,WIDE,WIDE If you select 4 you get WIDE, if you select 5 you get WIDE,WIDE and so on. THis separation into two distinct strings gives you the chance to have a path beginning with RELAY or beginning with WIDE. This is important for operating in areas which do not yet have the dual alias WIDE-RELAY digipeaters yet. Although the longest path is now limited to four hops, anything beyond 2 WIDES is frowned on anyway...

REPEATER TNC ROUTING: If the TNC is just a conventional TNC, then it digipeats simply according to the path included in the Mic-E packet. The special APRS NODE, however, has two routing methods, depending on whether the surrounding APRS packet network is capable of the WIDE-N algorithm or not. If WIDE-N is available, then the APRS REPEATER node simply digipeats the packet to WIDE-N where N is the number of hops indicated in the packet SSID. If N is greater than 7 then it is a directional packet and the North, South, East, or West paths stored at the NODE are used. If WIDE-N is not yet available, then the NODE builds a digipeater string using the same algorithm as the MicEncoder; offering the optional 3 and 4 hop alternatives for 1 through 7 and still uses the directional paths for numbers greater than 7. ALso in the directional path, if the 3rd SSID bit is set, then a WIDE is added to the end of that path.

WIDE-N ROUTING: In WIDE-N mode, every APRS digipeater repeats every WIDE-N packet it hears and then subtracts ONE from the SSID. They also keep copies of all such packets (or just keep a checksum) for 60 seconds and ignore all DUPES of the same packets. THis is a very effecitve OMNI routing method that permits the packet to go out N hops in all directions without any duplication!

IMPLEMENTATION: The key to the success of the MIC-ENCODER is that it is very versatile and can operate in all required modes. This allows for growth and improvment in the APRS systems without obsolesence. There are five possible operational situations as follows:

MIC-E path is set to 0 and anyone monitoring the repeater output with APRS can track users.
(But TNC @ someones QTH)
MIC-E path is set to 2 (RELAY,WIDE) and someone's home station monitors the REPEATER output with his TNC and DIGI's the packet over onto APRS
STANDARD TNC @ REPEATER: MIC-E path is set to 1 thru 7 in DIGI mode. The TNC with the alias of RELAY (or WIDE) repeats packets onto the APRS packet channel.
APRS NODE @ REPEATER: NODE routes according to MIC-E SSID bits only.
DIGITAL APRS CHANNEL: MIC-E path can be 1 thru 7 in DIGI mode.
The BIG difference between the MIC-E DIGI mode and the SSID mode is the length of the packet due to the DIGI fields. THis means for interim compatibility, MIC-E users can operate generally with a path of 1,2, or 3 on all possible channels without specific configuring. Of course, if they select SSID mode while on an APRS NODE repeater channel, their packets will be much shorter because they will be routed by the SSID alone.

SETTING DIGI or NODE MODE: To save front panel space, the high order bit of the MESSAGE switch is used to select between the DIGI mode and the SSID mode, the Mic-E then checks this 4th bit to determine how to send each packet. THis way a user can change between SSID and DIGI mode to match the configuration of the particular repeater he is currently using.. The front panel markings show that with the switch in the right side places you in DIGI mode, and the left side in SSID mode.

MIC-E PACKETS: The Mic-E always sends a compressed position report in every packet. If there is no position, then the Lat and Long are 0. If Telemetry is enabled, 5 bytes of telemetry are added, and if BText is enabled then the text is appended onto the end of the POSIT. The telemetry is captured on the alt-TELEMETRY screen in APRS and the BText will display on the LATEST STATUS page. NOTE: Since the BText may also contain a fixed POSIT in some applications, whenever there is a BText, then the compressed posit is ignored in that packet.

MIC-E SETUP AND CONFIGURATION: The MIC-E is based on the APRS Micro- Interface-Module (MIM) designed by Dr. Carl Wick (N3MIM) and produced by Dr. Will Clement N3LXR. The MIC-E is configured via its serial port using a PC program called MIC104.exe. This program provides a nominal TNC type command mode for setting the MIC-ENCODER configuration giving the user the standard cmd: prompt. Once the MIC-E is configured, you use the PERM command to cause the MIC-E to save the configuration in EEPROM. The following items can be configured:

MYCall Sets the MIC callsign
MYSymbol Sets the APRS symbol character
VIA digi1,, etc Sets the Unproto digipeater path
TXDelay Sets the key up delay for AUTO packets
TXDPtt Sets the key up delay for Mic-E PTT packets
PERiod Sets the nominal MIC cycle period
POSIT N Sets POSIT period as N * cycle period
TELEMETRY N Sets TELEMETRY period as N * POS period
BEACON N Sets BEACON period as N * POS period
AUTO N Sets AUTO period as N * POS period
QUIET N Sets the QUIET period as N * cycle period
BText Sets the Beacon Text
PTT (1:0) Sets sense of the PTT signal. For the MIC-E, this is 1 since an external PTT transistor is used.

GPS INPUT STRING: Currently, ONLY the $GPRMC is supported since it has both position, course and speed.


POSIT NOW! This function was added in Mic-E104e so that you could override the timers and transmit a packet at any time. Just turn the PERIOD pot momentarily to 0 (minimum). This forces a posit NOW. You may leave it at minimum as long as you want (my usual position) since it will only send ONE packet. If you are already at 0, just rotate it up a bit and then back down. In the Mic-E104e betas there is a jiggle zone about 1/8th of a turn up where you may end up with CONTINUOUS packets since we forgot to "de-bounce" the transition across the decision boundary... Sorry...

USE OF MIC-ENCODER BEACON TEXT: The Mic-E's BText is included on the end of a posit report. But due to APRS processing on receive, only the BText will get through and the posit will be ignored. This is why you should always set your BText rate at a lower rate than your POSIT rate. For NON-GPS equipped MIC-Encoders, a null posit 000000/000000 will normally be transmitted. For fixed station use, however, you can put your full LAT/LONG in the BText, and APRS will get the posit from that. Use the format of BT !3859.11N/07629.11W$000/000 Where $ is the usual symbol character. If you choose to do this, then you can put no other text in the BText, or the MESSAGE bits will end up not being properly parsed.

A good example for this, is as a burgler alarm at a fixed location. Put the location in the BText as above, and connect the MESSAGE bits and PERIOD bit to contact closures in your alarm system. Have AUTO ON. When the bits get tripped, the message rate increases by a factor of 16 and the message changes to EMERGENCY or PRIORITY...

TELEMETRY: The MIC-E can also send 5 channels of analog telemetry. Mic104 and prior versions would send 4 channels of telemetry representing the MIM inputs AD0 through AD3. In Mic104d the PERIOD pot was moved to AD3 and AD1 became the SSID/DIGI select bit. But for compatibility with the MIM which could still operate with 5 channels, it was decided to always transmit the 5 channels in the MIC format even if AD1 and AD3 were already committed. This change from 4 to 5 channel format results in the following situations:

Pre-MIC104d > APRS77f: Telemetry page will show 4 good channels. 5th channel will show 0 or first byte of BText 1st char of BText will be lost when TLM is xmitted

MIC104d > pre-APRS77f: TELEMETRY page will show 4 channels.
2nd chnl will be 0 or 255 depending on SSID/DIGI
3rd chnl will be value of PERIOD pot
5th chnl will show --- and become 1st char of BText
DUAL USE ANALOG INPUTS: Since all 5 channels are now always transmitted, the telemetry page will show whether the user is in DIGI mode and what is the current setting of his PERIOD pot. In addition, since the AD1 only indicates DIGI mode if the analog value is 7 or below, this analog input can still be used for analog sensors with outputs between .16 and 5 volts while in SSID mode. A 47K pull up resistor assures a positive value when the SSID bit is not grounded. Additional user momentary push button actions are anticipated as dual use applications on the other analog inputs as well.

WARNING: Unless the analog inputs are tied to a voltage through an impedance of less than 10K they may show some crosstalk to adjacent channels. If they are floating, they will surely show random values. Or in the case of the dual use, the 47K pull up resistors will show strong positive values near the maximum 255.

PTT LED INDICATOR: The PTT LED is connected to the MIC PTT output line that shows when the Mic-E is pulling the PTT low. This shows the user that a packet is pending and will be sent when he releases the PTT.

RECEIVER LED: This LED is connected to a simple audio rectifier of the receiver audio. As long as the LED occasionally flickers, the MIC-E will not AUTO-initiate any packets. If the LED has not been driven by the receiver for the period of the QUIET timer, then an AUTO packet is initiated. If AUTO is set to OFF, then the switch permanently lights the LED and prevents all AUTO-packets. Users must set the receiver audio high enough to tickle the RCV LED frequently in order for holdoff to work.

WARNING!: If you turn the audio down because the wife wants quiet, then your Mic-E will not have a holdoff signal and the AUTO timer may transmit on the repeater over other users if AUTO is enabled!

RADIO INTERFACING: THere are three ways to wire the MIC-E to your radio system depending on your preference:

A) Plug your Mic into the Mic-E and plug the Mic-E into your radio
B) Wire the MIC-E in parallel to your mic either at its connector or internally to the radio or to an auxiliary input (But the MIC-E must be able to sense the MIC PTT independently)

Option A was chosen as the Mic-E production method since it allows for universal jumpers inside the Mic-E and no user soldering of Mic Cables. Its disadvantage is that the Mic cord tuggs on the Mic-E and this limits mounting options for the Mic-E and usually requires the Mic-E to be hard mounted in the vehicle. B requires more inventive hacking for the user but allows the Mic-E to be mounted anywhere in the vehicle.

Note that option B requires cutting of the PTT lead so that the Mic-E can sense the Microphone PTT before it gets to the radio as follows:

   MIC JACK                                                 MIC PLUG

short 6" pigtail To Radio

From Mic >--green-------------> Up Button >--violet---------------> Down button >--brown/shld------------*-> PTT/Radio gnd >--orange----------------|--*-> Rcv audio >--black-----------------|--|--*-> 5/8 volts DC >--yellow----------------|--|--|--*-> Mic Audio >--blue------------------|--|--|--|--*-> Mic Gnd >--red----------------* | | | | | *-> PTT to radio | | | | | | | | | | | | | | PTT into MIC-E 3 <---red------* | | | | | | PTT/Radio ground 2 >---brown/shld--* | | | | | Receiver audio 4 >---orange---------* | | | | 5/8 volts to Mic-E 1 >---black-------------* | | | Mic Audio 5 <---yellow---------------* | | Mic Ground 7 >---blue--------------------* | Mic-E PTT output 6 >---green----------------------* 8 violet not used (I would parallel it to GND) Mic-E CONNECTIONS


First you must determine how to power the MIC-E. If you will be using a GPS with its own power arrangements, then you may power the Mic-E from the Mic jack power. If you will be using the internal GPS or wiring GPS power at the same time, you may as well use external power for the Mic-E.


External power: Place JP13 on 1-2 and place J14 on the 1-2 position.

Mic-power: First, measure the voltage at your MIC jack with a 330 Ohm load. The resulting value will decide the remaining steps:

  1. If the voltage is above 7 volts then install JP13 in the 1-2 position. and JP14 in the 2-3 position and install JP1. This will route the power through the voltage regulator

  2. If it is less than 7 volts but more than 5, then install jumper JP1 and jumper JP13 in the 2-3 position and JP14 in 2-3 position. This bypasses the regulator.

  3. If it is exactly 5 volts and appears to be well regulated, then Perform step 2 above AND install JP6 to bypass the series 10 ohm dropping resistor.

Arrangements 2 and 3 will use only a Zener to regulate to 5 volts. BE CAUTIOUS, HOWEVER, because this input has the regulator bypassed. CHECK ALL OF YOUR RADIOS and make sure you will not over power the ZENER. Incorrect jumpers can easily blow all IC's on the Mic-E.


Interfacing ANYTHING to your microphone circuit is not trivial. Any ground loop will add noise to the MIC audio (remember the alternator noise problems...) Drawing 15 ma from the MIC circuit adds to this problem too. Separately powering the GPS from the 12 volt system and then connecting that data ground to the MIC encoder is also a potential noise source. Be sure to use the isolated MIC ground as shown in the circuit. Do not just connect this wire to just any-ole ground! If you do, circulating ground currents will degrade the packet audio. If your packets do not sound clean, you may have to power your MIC-E with a 9v battery to get clean power and audio...


1) The MIC-E was only designed to work with radios with separate PTT circuits. Many HT's with combined PTT/MIC audio lines will not work.

2) The Mic-E BETA versions were not at true RS-232 voltage levels. The data only swings between 0 and 5 volts on output and may be incompatible with some RS-232 serial ports. It should work fine with the GPS however. If you are having consistent link failures with the MIC104.exe program try this:

            Mic-E output >-------//\//------*------> PC input
                                   1  k       |
                          -  *---//\//------*
                            ---    10 k
                            ---  9 volt battery
                          +  - 
                             *-----------------------* Ground


JP1 REGULATOR BYPASS - With this jumper ON, you can bypass the regulator chip when available MIC power is less than 7.2 volts. Use CAUTION. DEFAULT is OFF. See JP6. JP2 AD0 INPUT - With jumper on pins 1 and 2, AD0 will read supply voltage in tenths of a volt. Meaning 126 = 12.6 volts. With jumper on 2 and 3, AD0 reads external voltage on pin 1 of the extternal connector.

JP3 Not used on MIC-E prototype.

JP4 GPS INPUT - With the jumper in place, the GPS can be programmed externally from the DB-9

JP5 TBD - Default on pins 1 and 2.

JP6 5 VOLT POWER - This jumper bypasses the 10 ohm series resistor in the Zener regulator circuit and relies on the source impedance of the MIC circuit power for current limiting. WARNING: Be sure to never use this jumper when supply voltages are above 5.8 volts. DEFAULT is OFF.

JP7 TTL INPUT - With jumper on 1 and 2, you may input TTL data at wire point 12. On position 2 and 3, data input is quasi RS-232 on pin 2 of the DB-9 connector. DEFAULT is 1 and 2.

JP8 GPS INPUT - With Jumper on, the internal GPS-20 is connected to the MIC Serial input port. With jumper off, you may use the DB-9 for external serial connection to your PC for configuring the MIC-E.

J3 Mic-E LOADING - This jumper allows you to minimize the impedance loading of the Mic-E on your existing Mic Circuit. Use the highest value resistance that still gives suitable packet audio level without loading down the voice audio. DEFAULT is on pins 3 and 4 for 10K.

OPERATIONS: My initial guess at a POSperiod is about 1 minute and an auto period of about 4 minutes and a QUIET time of about 10 seconds. I usually leave AUTO OFF so that I wont key up the repeater unnecessarily. I also always have PATHS set to zero so that the packet is as short as possible. Since no one is really tracking me yet, to put out a good posit on the APRS network, however, I just dial in my 145.79 memory channel, set PATH to 3 (RELAY,WIDE,WIDE) and kerchunk the mic, and listen for the digipeat. If I hear it, then I got in and I'm on the maps!.

AUTO has two uses. On a voice repeater you might set QUIET to long enough to be sure the repeater is really not BUSY before the Mic-E fires off an AUTO packet. But if you use AUTO on the APRS packet channel, then you want QUIET to be ZERO so that it acts only for colision avoidance. If QUIET is nonzero and you have a busy APRS channel it may never transmit!. Override this if you need to, by simply turning the volume down, but then you dont have colision avoidance. So this is a tradeoff.

Once the REPEATERS mute the packets, then QUIET might be able to be set to zero on the voice repeater too and this will solve this problem... Just be sure to never turn your radio volume down on a REPEATER if you have AUTO on, or you will BRAAAP other people...



This special TNC NODE is designed to be intgrated into typical amateur voice repeaters. The TNC performs a number of special functions to fully implement the APRS LOCATOR SYSTEM:

Notice that although the APRS REPEATER NODE function only listens on the voice repeater input and only transmits on the digital APRS packet frequency, it must also have a secondary carrier detect on the APRS packet channel to avoid collisions. This special APRS node function is NOT involved in any further routing on the APRS digital channel (I mean that it does NOT serve as a general purpose APRS digipeater on the digital channel). All it does is to insert the appropriate directional or OMNI digipeater path and digipeat the packet. This distinction, of course, is only a functional distinction, since APRS digipeater functions can be co-located, or even built into the same NODE box as long as dual digital receiver channels are maintained.

REPEATER MUTING: Since the acceptance of the POSIT-PACKET on voice repeaters will be determined by the minimization of the BRAAAAAAP sound on the repeater output, the DCD and subsequent muting of the repeater are important. Muting the repeater output is easy if use the signal from the DCD LED on the TNC to implement a 20 dB attenuation in the audio line to the repeater transmitter. I think a 10 to 20 dB attenuation is appropriate. If it is completely muted, then no one will know about your new toy, and more importantly, the DCD may occasionally mute some sylables of some voices. The problem is that the TNC must have the add-on True- DCD so that it responds only to packets and not voice. See the following section.

USING ANY TNC AT THE REPEATER: Unitl the special APRS NODE is available, you can use any TAPR-2 compatible TNC with the add-on True DCD such as the PacComm Tiny-2. Just connect its audio input to the repeater input receiver and connect its TX audio and PTT to a small 1 watt XMTR on 145.79. Use the output of the add-on True-DCD circuit to mute the voice repeater transmitter. This will work fine, but is not going to avoid collisions. It is better to have a transceiver on 145.79 and use its squelch to drive the external Squelch DC signal on the TNC to implement CSMA. Be sure to isolate the add-on True-DCD output from the Squelch output so that packets on the Packet channel do not also MUTE the voice repeater!

NOTE: You cannot use the KPC-3 TNC set to software DCD, since it then IGNORES the external DCD defeating this dual carrier detect capability.

NOTES: Notice that the APRS REPEATER NODE will also work on the digital channel! In other words, the APRS REPEATER NODE algorithms can also be running simiultaneously in all of the APRS DIGI's so that the APRS compressed format will be picked up directly on the digital channel. These original packets are distinguishable because they DO NOT have an original DIGI field. Once a NODE processes them and adds the DIRECTIONAL or WIDE routing, they will be forwarded as usual. Notice that the NODE hardware can actually do both functions as long as dual digital receive channels are provided.


The following partial schematic shows how the MIC-ENCODER evolved from a general purpose MIM module:

 MICROPHONE                                                      RADIO
 ----------                                                     --------

 MIC >-----------------------------------------*----------------> AUDIO
                                         47k   |
                                     *--//\/--*  *-------------> MIC gnd
                                     |            |     
 +8v <----------*--------------------------------------*--------< +8 V
                |                    |            |    \ 1k
                |              D1    |            |    /
 PTT >---*---------------------|<----------------------\------*-> PTT
         |      |                    | MIC audio  |    /      |
         |    |-|----------------------|          |    *      |
         | D2 | *                    * | MIC GND  |    V  LED |
         *-|<-|-*    MIM MODULE      *-|----------*   ---     |
   PTT in     |                        |               |  D3  |
              | *               *    * |  PTT OUT      *--|<--*
              |-|---------------|----|-|    5.6K     |/c  
                |               |    *----/\/\/\-----|   Q1 NPN
                |    *----*-----*                    |\e
                |    |    |     c\|  Q2 NPN            |
                | S1 *    |       |--------*-----||-------------< RCV AUDIO
                |  \      |     e/|        |   .2 uf   |
                |    *   ===    |          -           |
                |    |    | 6uF |          ^ D4        |
                |    |    |     |          |           |
 GND >----------*----*----*-----*----------*-----------*--------> GROUND

CIRCUIT DESCRIPTION: D1 isolates the microphone PTT from the radio PTT input so that the Mic-E can key the PTT line while also sensing the MIC PTT condition. D2 prevents the MIC-E from grounding the PTT lead when the MIC-E is turned off. Q1 is an open collector PTT transistor. The LED shows when the Mic-E is holding a PACKET, waiting for the user to release the MIC PTT. D3 isolates the LED from the mic PTT. The 47K minimizes circuit loading and the isolated MIC ground is connected to the lower end of the MIM audio pot to minimize ground loop noise. Q2, D3 and the lower two caps are an audio rectifier to drive the HOLDOFF input to the MIM when the radio is in use. S1 is the AUTO switch. When OFF, it asserts hold off, so that the MIC-ENCODER will never auto-initiate a posit on its own. Bits D2 to D8 connect the PATH and MESSAGE switches and A5 is the PERIOD adjust pot as follows:
                                             A3 (was A5 in early MIM/MIC's)
               D8 D7 D6 D5 A1 D4 D3 D2       A5    +5v
               *  *  *  *  *  *  *  *        *      *
               |  |  |  |  |  |  |  |        |      |
               -----------------------       |      |
               |          |          |       |      /
               |     7    |     7    |       *-----> 10k
               |  P A T H |   M S G  |              /
                     |          |                   |
                   -----      -----               -----
                   ////       ////                ////

SERIAL PORT FOR GPS AND CONFIGURATION: All users should consider using the APRS standard 1/8 inch stereo phone plug/jack for their serial data port on the MIC ENCODER and other small stand-alone-trackers. The phone plug is small, readily available, and is compatible with the nominal mono 1/8 inch phone plug found on many GPS units:

    GPS UNIT or PC LAPTOP                 MIC-E or embedded TNC
    ---------------------                 ----------------------
    mono or stereo PLUG                   1/8th inch stereo JACK
    TXD (data out)   --------------------> TIP    
    RXD (data in )   <-------------------- RING   
    GND              *-------------------* SLEVE
To help remember, just think of the DATA comming out of the male plug tip. This applies to the GPS by itself or to the laptop used to CONFIGURE the stand-alone tracker. By using a "closed-circuit" jack, an internal GPS can be normally connected to the internal MIC-E or TNC, but plugging in the LAPTOP opens that circuit and connects the laptop to the TNC... TEMPERATURE MEASUREMENTS: By proper selection of 2 resistor values and 2 to 4 diode voltage drops, you can easily make one of the Digi-Key $2 thermisters read out temperature in degrees F. For details, run the MIC-TEMP.BAS program. It is suggested that AD-2 be used for internal temperature just for consistency with the default APRS Telemetry Display.

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