Thursday, 7 September 2017

Simple 70cm Fox Hunt Reciever

After building the Simple transmitter I decided to use the companion reciever module to the FS1000a emitter used previously, again the concept was to keep everything simple and easy for children to construct (under supervision).

The reviever is contructed around the MX-RM-5V module which operates on the 433.92Mhz ISM band. Again using parts from the junk drawer I came up with this simple circuit.

The circuit runs from a 6v battery pack (4x AA Cells) and utilises both audio output via headphones and visual feedback from an LED. The main component is a BC547 NPN transistor, however pretty much any NPN transistor could be used. Output from the transistor provides just enough current to drive the headphones without the need for an audio amplifier but a simple single IC solution like the LM368 could be utilised. Now I must note the 433.92Mhz ISM band can become extremely congested and this will reflect on the receiver, however as we plan to use it in a large country park it should not be much of an issue.

I originally wanted to build a simple 70cm Yagi from a tape measure, however decided the sharp edges were not child friendly so I came up with the idea of using copper self-adhesive tape on a pre-constructed frame. The frame is built from 15mm pine and contains a director, driven and reflector element. The design shown below used pre-cut pieces that can be sandwiched to produce the yagi frame.
Each element is 400mm long, the inital spacing is not important however the 100mm and 80mm spacings between the director, driven and reflector elements is important. The overall length of the frame is also users choice but should be long enough to accomodate the pcb/battery and handle.

Copper tape is then stuck on to each element for the total length, this can then be cut to 'tune' the antenna. The middle 'driven' element needs to be cut in the middle to isolate the two halfs of the element and can then be soldered directly to connect the reciever. The follow lengths are what worked best for me, but feel free to play with them;

Director:   306mm
Driven:     312mm 
Reflector: 360mm

I used a cheap plastic lunchbox to house the electronics and co-axially mounted the battery pack, this doubles in purpose as the unit lacks a power switch. The clear plastic of the box also negated the need to externally mount the LED.

If anyone has attempted to build this project or improved on it I would love to hear about it!