There is a popular activity in ham radio called Fox Hunting. This is where you hid a small transmitter in an outdoor location and then the hunter uses a radio direction finder to locate the transmitter. Kids and teenagers really enjoy this and it’s a fun introduction to the radio hobby. 

To build a simple beacon transmitter I decided to use an inexpensive toy walkie talkie module called the SR-T300, which I purchased online. This module only needs a battery, speaker, microphone and a couple buttons to become a complete walkie talkie. However I only wanted it to transmit a beeping sound, which would be perfect for this activity. 

Wanting to keep it as simple, small and inexpensive as possible. I chose to use a 3mm 1Hz blinking LED as the timer and assembled it nose to nose with a phototransistor. The SR-T300 has a call function, which just transmits a beep when that pin is connected to +3v. So I connected the phototransistor to that pin. I encased the phototransistor and the LED with shrink tubing and some black acrylic paint to prevent interference from ambient light. I also connected the PTT (push to talk)  pin to +3v so it would transmit continuously while connected to the battery. 

Then I attached a CR2032 battery holder to power the unit. At 3 volts it transmits a strong signal over several hundred yards.  All together it’s very small and easily fits inside a pill bottle or a plastic Easter egg.  The 1Hz beep makes it easy to identify. 

Usually Fox Hunting is done with VHF units, the module I picked up transmits at 433.5 Mhz within the amateur radio UHF band.  The advantage of using UHF is that the receiving antenna is much smaller and can be made more accurate. 

I’m very pleased with this unit and have used it several times at local ham radio events.