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Radar Detector Tests

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Measuring Laser Gun Pulse Rates
A frequent question we get asked is: how can I measure the pulse rate of my laser gun?

Here is one quick and dirty method, that uses only a few components. Here is what was used for this example:
  • Radioshack Digital Multimeter #22-811
  • SFH203FA photodiode
  • .1 uf Capacitor
  • 10K Resistor
  • 9V Battery

The Schematic Diagram

The completed circuit.

The circuit in action.


If the circuit and meter are the exact same as tested above, then it should work to measure the pulse rate of most laser guns. However, many substitutions are possible.
  • Of course, just about any freq counter or meter with counting ability should do the job, with some caveats (see below in the troubleshooting section).

  • The SFH203FA photodiode is optimal, since it is centered at 900nm and includes a built-in spectral filter. This same photodiode is used in some laser detection circuits of commercial detectors/jammers. However, many different photodiodes can be used, with varying degrees of success.

  • The capacitor can be omitted as long as you are sure your meter or counter can handle the DC power you are using.

  • In most cases the resistor can be left out of the circuit, depending on your power source.

  • The freq counter characteristics are important for success. The circuit above works great with my bench freq counter, and the Radio Shack meter used above. But, I have a more expensive handheld freq counter, and the frequency jumps around quite a bit, although it is stable enough to give you a good idea what the pulse rate is. Reportedly, some Fluke meters will do this as well. Adjusting the gate time might help on some counters. Interestingly, the Radio Shack meter above is the best I have used so far: the frequency is always stable and dead-on.

  • If you aren't getting good results, try increasing the power to the circuit. The SHF203FA can handle up to 50V reverse bias voltage. If you go as high as 50V, I would double check to be sure the capacitor is rated for it to protect your meter.

  • If you're not getting a reading, try moving the photodiode around in front of the transmit aperture to find a good position: the laser beam is very thin close to the gun.

  • Don't move it too far away from the laser gun, it will only work at close range. This circuit is meant to get the job done using the fewest components possible, and isn't meant to be a "laser detector".


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