Sensitivity
The receiver module needs the LM386 audio amplifier chip as shown below. The CommLinx RLP418 (or the Velleman RX418) AM receiver module specs shows about 3µV sensitivity and it is designed to operate with a supply voltage of 5v DC. Although it has less sensitivity of the Lemos FM receiver (0.5µV), the schematic below works very well with it and provides reception at 3/4 mile on the ground and many miles in the air when used with a homemade yagi antenna like the one at Programming Pete's. Good communications receivers are tunable, have better sensitivity and selectivity, and provide better performance. After tuning the receiver to match the transmitter, I've received a good signal over 1 mile away on the ground.
NEW DEVELOPMENTS: I've been investigating new reciever modules to use with the my circuit board. They are the same size and has the same pin-outs as the old modules. It will be necessary to unground pin 6 for some of these modules. But, they have significantly better sensitivity specifications and provide more range than the old ones.
Receiver Construction Notes
The input wires should be tightly twisted. The potentiometer is a 10k audio taper. You'll get the best performance using a 0.1 watt speaker or low power headphones like those for pocket CD players. Headphones are usually not needed and awkward to use. However, wiring a headphone jack so that it switches in headphones instead of the speaker is very useful for high wind or noisy conditions.
Using unshielded/untwisted leads and/or using the receiver module with a 9v battery and no voltage regulator causes circuit feedback. So, don't eliminate them. This circuit has more than enough audio gain provided that the LM386 is added.
One of my receivers using the older receiver modules (RX433) did not work at all until I adjusted the slug of the coil on the receiver module. After initially adjusting the coil to get a signal, I later put the transmitter in a box and shielded it to make the signal as low as possible from another room so that I could fine tune the receiver for maximum sensitivity at the correct frequency. Regardless of what the advertising specifications say, it may be necessary to fine tune the slug on the RX433 receiver module for best performance. This must be done with a non-conductive tool. Also, the transmitter frequencies vary some from module to module. (I found significantly lower range on one module and traced it to the frequency being off by about 200khz. That module was replaced under warranty.)
| photo-etched PC board | ||
| RX418 | RLP418 or (RLP418a) receiver | CommLinx or Velleman |
| IC1 | LM386M-1-ND IC AMP AUDIO POWER LOW V 8-SOIC National Semiconductor | |
| IC2 | LM2936Z-5-ND IC REG ULTRA-LOW CURRENT or 7805 voltage regulator 5 v |
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| C1,C2 | 0.1 microfarad ceramic capacitor | |
| C3 | 10 microfarad electrolytic capacitor | |
| C4 | 220 microfarad electrolytic capacitor | |
| R1 | 10 k audio taper potentiometer | |
| SPK | 8 ohm | |
| BNC | panel mount BNC connector - female | |
| 9 v battery connector |
0.1 watt speakers from Radio Shack can be damaged by using receiver at high volume. (If your audio gets progressively weaker and goes out, you've been turning it up far too high and have fried your speaker.) Walkman-style speakers from a broken headphone are slightly thinner.
The mounting holes on the circuit board are large enough to allow the use of a much larger 5 volt regulator if you desire to use it instead of the ultra-low quintescent current 5 volt regulator.
* Twist supply lead and supply ground very tightly.
* Use shielded speaker wire or tightly twist speaker lead and ground.
* Going to and from audio taper, use shielded microphone wire with two center
conductors or twist leads very tightly
* The wires between the BNC connector and the circuit board should be very short (i.e. 1/2").
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Enlarged image of circuit board. The circuit board is actually about the size of a 9v battery. Green represents the foil on the bottom side of the board. Red is foil on the top side. The actual board contains no labeling. Holes are plated-through. In the box, you'll need room for a headphone jack, a potentiometer with switch, an earphone speaker, a BNC connector, the receiver board, and a 9v battery. . |
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Component Hookup Diagram The audio output in the image above is shown connected directly to a speaker. As indicated earlier, it is sometimes useful to add an audio jack for Walkman-style headphones. Because of the differences in appearance of the headphone panel mount jack connections, I did not include an image of the hookup of the jack. p.s. If the volume seems to operate backwards, check the 3 wires going to the potentiometer. It is not uncommon for people to accidentally swap the connections going to PT and PG. . |
* Remember to use 50 ohm coax rather than 70 ohm tv coax when connecting BNC connector to your antenna.
The parts are identical except that the board above uses a surface mount audio amplifier while the one below uses an standard 8 pin audio IC.
view from top (component side) of circuit board
Circuit board prints the correct size at about 600 dots per inch. Be sure to check it with the receiver module and adjust the printing size if necessary. You can etch 8 complete receiver boards on one 6" x 6" board.
The following is an introduction to circuit board production. For drilling circuit boards, a 1/32" drill bit, Dremel Tool (with chuck), and magnifier are used
| MG Chemicals
Photoresist Process This three step process produces a professional quality circuit board. |
