Inexpensive Tracking Electronics

PIC-basedTransmitter Board

Transmitter Specifications

Output Frequency: 315 mHz, 418 mHz, or 433.92 mHz (corresponding receiver modules are used)
Peak Output (specified): 10 mw
Modulation: AM

Maximum Supply Voltage:  12v
Minimum Operational Voltage:  3v

With a fresh 9v transistor radio battery battery, you can expect about 8 mw output.  When under a 10 ma load, voltage of 9v alkaline batteries drop off at a consistent rate from 9 volts to 6 volts.  Current and transmitter output drop off at higher rates than battery voltage.  Transmitter life with a fresh 9v Duracell alkaline transistor radio battery is about a week before the battery voltage drops to 6 volts.  Out of curiosity, I tried powering a completed PIC-based transmitter with a total of 3 volts and was surprised to find that PIC and transmitter were still operating although with extremely low transmitter output.  (This is about 1 volt lower than the PIC minimum voltage spec.)  I've not tested the range with 3 volts but suspect that it will have dropped to about 30% of the range with 9 volts.

Use of a small 12v alkaline remote control battery results in increased output and range.  However, 12v battery life is only a few hours.

The following table is for a different transmitter module.  Measurements of current on my assembled transmitter are similar except that the consumption is less than 1 ma during the spaces between beeps because the transmitter is off during those periods..

Parts

  transmitter board (sub-assembly)  new 4 pin version
         tlprlp434a transmitter module specs   note: The circuit example shown there is not relevant to our application.
  LM2936Z-5.0-ND regulator
  PIC 12C509A
  10 uF 16V Tant capacitor
  Terminal block (2.54mm 4 pos)
  Custom double-sided drilled pc board with plated-thru holes

Construction Notes

Solder the surface mount capacitor before other components being careful to mount the side with the band on the positive.

Instead of inserting the transmitter subassembly board pins all the way in the holes, mount the assembly just high enough that it can be bent over flat against the circuit board after it is soldered.

(The PIC program supports different audio pitches.  To conserve space, I chose not to add dip switches on the pc board.  You can alter the pitch according to the following table by hacking the code in the PIC and by making wiring changes on pins A and B.  In order to alter the frequency from 740 Hz, I found it necessary to tie the ungrounded pin A or B to positive 5 volts.    I did this by drilling out the ground on the bottom of the selected pin on the board before installing the chip, then soldering a wire on the chip between pin 1 and the ungrounded pin.  Afterward, I installed the chip as usual.  None of that is necessary if you use the default 740 Hz tone.)

You may notice on the board that there is an extra ground and that there is a place to connect a wire to pin 5 of the PIC chip.  These provide an optional 5v power out switched by the PIC.  This pin can supply up to 25 ma.  The low current power regulator is only designed to handle a total of about 50 ma for the PIC when the rocket is at maximum temperatures.  This is more than enough for the transmitter at normal voltages but may be inadequate if you add an additional drain on pin 5 while simultaneously increasing the supply voltage to unusual levels in order to get more output from the transmitter.

Click on the quarter for large drawing showing all traces..
(Actual size is 1.8" x 0.8" - about the size of your thumb but shorter.)
  It fits with room to spare in a 24mm airframe.

PIC Program - used to control the PIC was written by Programming Pete.