What are the components needed for this receiver?

Question

I want to build a simple crystal radio receiver. I do not want amplify the signal received. I want to detect the presence of a transmitted signal, and then use the presence of the received signal as a trigger to switch on another device (battery powered). My knowledge is VERY basic, and I have been trying to learn more. I have found a schematic on the web.

http://www.circuitdiagram.org/how-to-make-build-crystal-radio.html

Is this design suitable for my intended purpose? (minus the Earpiece) From what I can tell, this receiver is made up of an inductor, a variable capacitor, and a diode. I am assuming my circuit would need a transistor switch as well. What else would I need? (I have assumed here that the antenna is part of the inductor coil - is that correct?)

I am looking for components I can use to make up this circuit. I want to receive the signal across a range of 60 feet and I want the circuit to be small (>1cm). What else do I have to ascertain in order to start searching for the components I need?

If anyone can to suggest suitable inductors, capacitors and/or diodes - this novice would be grateful!

Answer 1

Since you are trying to switch on battery power, you have battery power available. So use it.

Your design brief contains two completely incompatible requirements; powered from the signal, and small size. To get significant power from the signal, you need to collect significant signal; and that means a large antenna.

The classic AM band "crystal set" circuitry you are studying needs about a hundred foot wire antenna to collect enough power to run sensitive headphones. At which point you might as well run the wire to the transmitter!

At higher frequencies you can reduce the antenna size to some extent but if you need the antenna to be small you MUST expect the received power to be very low, and need amplification.

Even your transistor switch is an amplifier, so admit it; there will be amplification. Then focus on the least power you need to power that amplifier - off the battery. One way might be to use a very low power micro like the MSP430 (under 1 microamp in the right usage pattern) to turn the receiver on very briefly every few seconds, to see if a signal is being transmitted.

Answer 2

To answer this question, you'd first need to define what "a signal" is, and what it means to "be received". Consider that the world is full of electromagnetic noise, both natural and man-made, and reliably picking this signal out of the noise is terribly difficult. It's easy if you have a headset and a button and perform the detection in wetware, but doing the same in hardware is much more difficult.

You are missing some concepts to understand how to approach this problem. Here are some starting points:

• modulation
• the specific modulation types that might be used with a receiver like this:
  • continuous wave modulation
  • amplitude modulation
• selectivity
• LC circuit

Also as Brian Drummond points out, you will almost surely need some amplification to do about anything with this circuit. You can get some power just from an antenna, but it is very small. Most of the power from the transmitter will be lost due to the inverse square law and the fact that most of the transmitter's power radiates uselessly into space, not into your receiver.