I would like to build a low cost frequency detector to check whether or not a key FOB is transmitting or not. My understanding is that they transmit in the range of 300-400 MHz. Can anyone help me?

 

Either 315 to 318Mhz, or around 433MHz. Both frequency bands are used for this purpose.

How much money you willing to spend?

 

I'd like to build it low cost (under $20), but what have you got in mind?

 

Buy one each 315MHz receiver (like this).

Buy one each 433MHz receiver (like this).

Power both, and use a scope to view the data output pin(s) while keying the FOB. One or the other will show a data train. You could hook the data outputs to LEDs. I believe that the LEDs will flash at random while the receivers are not detecting a nearby transmitter. The flashing pattern should be regular, and noticeably different while receiving a valid transmission for a FOB.

 

my Icom ic3 portable reciever covers those frequencies and a lot more. .5 to 2450 mhz. there are portable devices available. or if you are just looking for a go - nogo, a germanium diode across your dvm probes will tell you if its transmitting. 1n34 or equvalent. a few milivolts dc on meter.