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Digital Call Sign Annunciator

Bare Voice Module (from Radio Shack)
HamCam-ID'er is a tiny digital voice playback device that is designed to announce your personal ham call letters every few minutes. Its small size is perfect for mounting on video equipped R/C model aircraft.

In the USA, R/C wireless video hobbyists are required to operate their video transmitters under the FCC Part 97 regulations (amateur radio). The only exceptions are the Part 15 approved systems, but these low power alternatives are rarely used. Other countries have similar requirements. Playing by the rules will certainly prevent conflicts with government officials, since they can levy substantial penalties to offenders. But more importantly, following the rules is the basis of good amateur radio manners.

One of the requirements that is easily overlooked is the need to identify your station during your video communications. From what I have seen, some R/C wireless video users are neglecting to broadcast their ham radio call sign. Essentially, USA hams must do so every ten minutes and at the end of their communication. Here is a excerpt from the FCC Part 97 document:

§97.119 Station identification.
(a) Each amateur station, except a space station or telecommand station, must transmit its assigned call sign on its transmitting channel at the end of each communication, and at least every ten minutes during a communication, for the purpose of clearly making the source of the transmissions from the station known to those receiving the transmissions. No station may transmit unidentified communications or signals, or transmit as the station call sign, any call sign not authorized to the station.

Speak no Evil

I had been using simple minded methods to identify my video station. My favorite was to point the camera at a flight box mounted placard that had my call sign printed on it. Sometimes I merely spoke my call letters into the video systems's microphone. Both methods worked, but I was often forgetting to land my R/C model aircraft every ten minutes to do these cheap tricks.

Of course there are commercially made automatic call sign annunciators. I thought about using an on-screen display (OSD) board. These gadgets will electronically overlay your call sign (or any other text) on the camera's video feed at timed intervals. However, I was not excited about their size, current requirements, and cost. The audio based versions that I found were designed for ham shack use, so size was a problem with them.

As a devote hacker, I decided to build my own digital voice based call sign annunciator. The result is HamCam-ID'er. It is small, low power, and provides automatic call sign announcement over your video transmitter's audio subcarrier. It automatically mutes your microphone (if used) so that the call sign announcement can be clearly heard. Material cost is under $20.

Digital is Sometimes Analog

A few years ago I designed a consumer product that needed to speak a message. The best sounding single chip voice technology at the time was offered by Information Storage Devices (Winbond). Even though that was awhile ago, this firm continues to take the prize for good audio quality at low cost. The only thing that has changed is the owners (competitor bought them in a big-fish-eats-little-fish sort of transaction).

The funny thing about their "digital" voice chips is that they are not digital at all. Instead, the ISD folks discovered a way to store analog information (voltage samples) in a silicon memory array. The stored analog data is retained even when power is lost, so their audio chips make great play-only devices (but they are recordable too). Their chips are very integrated and require only a handful of cheap parts to create voice enabled toys and appliances.

Radio Shack Module, #276-1323To simplify the project, I started with a ready-to-use Recording Module from Radio Shack. This $9.95 part (p/n 276-1323) appears to be based on the recently discontinued ISD1520 ChipCorder® IC. This low cost bare-board module offers twenty seconds of recordable sound. It is a fun gadget and is ready to use after installing a 9V battery. The photo on the left shows what you get for your $10. Of course we will need to hack it a bit.

PIC Your Parts

The Radio Shack Recording Module has a speaker/mic and a couple of push button switches. To use it in our automatic ID application all we have to do is connect the speaker leads to our video transmitter's audio input and figure a way to "press" the play button every ten minutes. Sounds easy, doesn't it? Well it is, sort of. But we do have to add some parts to complete the job.

The general idea is to wait ten minutes then trigger the ISD's play mode. I could have used some "555" timer IC's to get the basic functionality I needed. But I also wanted to mute the mic and detect the end of message signal. A pushbutton switch for manually sending the ID at the start and end of the communication was needed. A status LED seemed like a good idea too. After penciling some design ideas I decided to reduce the parts count by incorporating a tiny 8-Pin microcontroller.

The chosen microcontroller is from the vast offerings of Microchip Technology. Actually, your exact PIC choices have some flexibility. You can use a PIC12C508, PIC12C508A, PIC12C509, and PIC12C509A. Project Update: You can also use the easier to find PIC12F508 or PIC12F509 instead.

The PIC12C50x is not a "Flash" part, so you will need a traditional PIC chip programmer to "burn" the hex file's object code into the microcontroller. Be sure to select the configuration fuses during chip burning as follows (these are optional settings within your chip programmer's menus):
      WDT: Enabled
MCLR: Disabled
Oscillator: IntRC
Memory: Code Protected

By the way, after you program the PIC it will fail the verify cycle. Do not be alarmed -- everything is OK. Just ignore the "failure." Whatever you do, do NOT program the chip twice!

If you have trouble burning the PIC, then please check your programmer. Whatever the fault, it is not a CamMan hex file issue. The most common problem is that the user has forgotten to burn the PIC's four config fuses, as shown above. More programming information can be found starting here.

The only other active component is an 8-Pin OpAmp IC. It is used to condition the speaker output from the ISD module (differential to single ended conversion). It also provides 1V style line level amplification for an electret condenser microphone. Although you may already have a microphone on your video transmitter, you will need to use the one from HamCam-ID'er if you wish to retain the mic muting feature.

Although there are only two tiny IC's, there are several other passive parts (caps and resistors). This project is best tackled by an experienced electronic technician. If you have successfully built any of the other RC-CAM Electronics Projects then you should have no problem with this one. Entry level technicians should plan on getting some hands-on help.

The finished device is a power miser. It will nominally draw under 3mA. During playback, the current is 23mA. Eliminating the LED1 lamp will reduce that amount to about 15mA. The allowed voltage range is from 4.5VDC to 12VDC, so it should be compatible with all R/C wireless video transmitters that have an audio channel.

Stop, Look, and Listen

Before you do ANYTHING, you must spend some quality time with Radio Shack's ISD voice module. Now is the time to attach a 9V battery to the board and record your call sign announcement. If you wait until later you will be very sorry (because you are going to hack the heck out of the ISD board).

You have up to twenty seconds for your recorded message, but you do not need to use it all if you don't want to. I suggest that you cup your hands around the Mic/Speaker during record and playback to improve audio performance. If you don't do this the sound level will be too low for good clarity. Practice a few times to get a feel for what sounds best (good volume, clear voice quality).

Here is a sample of what a ham ATV operator on 2.4Ghz might want to say:

" This is station Z Z 5 D U K . That is Zulu-Zulu-Five-Delta-Uniform-Kilo. Experimental ham ATV station operating on thirteen centimeters. "

Once you get your award winning announcement recorded you may proceed. So, if you are ready then let's get the soldering iron warmed up.

Springtime Pruning

Parts that need to be removed - Click for larger photoThe voice module will need to have some parts pruned from it. Franky, everything that dangles needs to be removed. Use a temperature controlled soldering iron (700° F) and carefully emancipate the speaker, battery clip, and record button. The play button is no longer needed, but I left it on the board.

When you are done with your pruning you will have some interesting spare parts. Keep them in a safe place! You never know -- you may need to record a new announcement one day (not a welcome situation, if you know what I mean).

HamCam-ID'er Construction:

The control board can be built using nearly any technique you wish. I needed it to fit inside the PacTec box used in RC-CAM4, so size was not a big issue (the enclosure is roomy). A piece of phenolic perfboard was cut to the same dimensions as the ISD module (1.75" x .95"). My method resulted in double-decker "daughter board" arrangement. Smaller dimensions are possible.

Perfboard Construction: Click for larger photoMy control board was point-to-point wired using the component leads and 30 AWG insulated Kynar wire. This wire is normally used for wirewrapping, but works fine with a soldering iron. I recommend a temperature controlled iron (700° tip).

Layout is not critical. I used sockets for the IC's (the empty one is for the PIC). For convenience, several resistors are mounted on the bottom side on my board. I suggest 1/8 watt resistors to keep things compact.

The schematic's J1, J2, and J4 components represent the connection points to the ISD voice module. The J3 connections go to the video transmitter's audio input. There is no need to add connectors to these areas -- direct wiring is best.

Connect the custom control board to the ISD module as follows:


Control Board

ISD Voice Module

SPK+ Audio


ISD Speaker Top Pad
SPK- Audio


ISD Speaker Bottom Pad
Play Signal


ISD R3 Top Pad
End Of Msg Sig


ISD LED Right Pad
5V LDO V-Reg


ISD R3 Bottom Pad


ISD C1 Neg Pad
Battery +


ISD BAT+ Input
Battery -


ISD Gnd Input

Connection Points to ISD board are shown

Check your work carefully. Do NOT install the PIC chip until you have verified that U2 pin 8 is ground and the pin 1 has 4.5 to 5.25 VDC on it when a battery is connected. Remove the battery BEFORE you install the chip. Simple mistakes can destroy electronic parts and may generally ruin your day.

The audio output from the control board (J3) is a 1V type line level. It will directly connect to any video transmitter that has a line level conditioned audio channel (a common feature). If your transmitter has a built-in microphone then you must remove it and connect its wires to the board. However, in this situation you MUST attenuate the signal. The schematic shows an optional L-Pad circuit that you should use to reduce the line level signal to the mic input's millivolt level. Resistor R16 can be adjusted if you need to change the volume (higher value = louder audio).

Here are the audio connections to route the control board's audio signal to the video transmitter:


Control Board

Video Transmitter

Line Level Audio


Tx Audio Line-In / Mic-In
Audio Gnd


Tx Audio Gnd (optional)

Testing, 1, 2, 3.

Now its time to test your work. This is a wise thing to do before you mount the ISD module and its companion controller board in the enclosure. Here are the necessary test steps (if any step fails then stop and troubleshoot before proceeding):

  1. Connect the battery. The LED will blink one time (on, then off) the moment powered is applied. If you do not see the LED blink once then something is wrong.
  2. Turn on your video system. Ensure that all A/V cables are connected. Verify that video is present.
  3. Speak into the microphone. You should hear your voice on your receiver's speaker.
  4. Press the S1 play button on the controller board (not the one on the ISD board). The controller's LED1 announce indicator should turn on and you should hear your recorded message on the receiver.
  5. The LED should turn off at the end of your recorded announcement. If it stays on for a full twenty seconds then something is wrong. It should turn off at the same time your recorded message ends.
  6. Press the S1 play switch again. While your digital voice is playing, tap or speak into the microphone and verify that it is disabled. At the end of playback the mic should function again.
  7. Wait exactly ten minutes from the time you pressed the S1 switch. Your recorded voice should play automatically. It will repeat at ten minute intervals.

Let's Wrap it up

Now you can install HamCam-ID'er in your enclosure. Mine shared space with my Panasonic board camera (see RC-CAM4). I used some double sided tape to join the controller board to the ISD module. Because of the harsh R/C model environment, I strain relieved the wires and verified that all the components would survive high RPM vibrations. I even glued some in place, including the two socket mounted IC's. Some R/C Rx foam padding was used to protect the boards in the Pactec Box.

Both Boards Sandwhiched Together - Click for larger photo.

Both Boards Sandwiched Together

Mounted in the Pactec Box - Click for larger photo

Mounted in the Pactec Box

Finished unit shown with RC-CAM4

Design Documents:

The technical details are available as file downloads. There is no charge for the information when used in a personal (hobby) project. Commercial users must obtain written approval before use.

Please be aware that the information is copyright protected, so you are not authorized to republish it, distribute it, or sell it, in any form. If you wish to share it, please do so only by providing a link to the RC-CAM site. You are granted permission to post links to the web site's main page ( Please respect this simple request.

Schematic Files Schematic Files: PDF file of the HamCam-ID'er circuitry. All components are from
Revision: Rev A, dated 03-18-2002
PIC Object Code Files PIC Object Code: Hex file of the compiled HamCam-ID'er firmware. You should occasionally check for updates.
Revision: V1.0, dated 03-18-2002.

The Small Print:

If you need a part then please consult the sources shown in the project (see schematics download). I do not work for, nor represent, ANY supplier of the parts used in HamCam-ID'er. Any reference to a vendor is for your convenience and I do not endorse or profit from any purchase that you make. You are free to use any parts source that you wish.

All information is provided as-is. I do not offer any warranty on its suitability. That means that if you build and use this device, you will do so at your own risk. If you find software bugs then please report them to me.


If you have technical questions or comments about this project then please post it on the rc-cam project forum.

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