Key FOB Tester

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
I still need an answer to my question. I need a way to test a repaired key FOB to see if it is transmitting. I don't care about codes or anything else. I would also like to know what frequency they work at. I am guessing in the range of 300-400 MHz.
 

blocco a spirale

Joined Jun 18, 2008
1,546
Most work at 433MHz in Europe (maybe 315MHz in the US), but don't tell anyone.

You can buy little Tx/Rx boards very cheaply from most electronic suppliers or ebay. The boards require 5V and present a demodulated output on one of the pins that would probably drive a low current LED.

You could have several boards, one for each of the standard frequencies.



It appears that we have been here before:

http://forum.allaboutcircuits.com/threads/need-to-build-an-rf-detector.78567/
 
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tracecom

Joined Apr 16, 2010
3,944
Coincidentally, I have just been playing with a 4-channel key fob transmitter and receiver. The receiver is shown on the left in the picture and three of its four channel outputs are connected to three anodes of an RGB led. When I push button A, I get red, etc. (The white coil is the antenna, which I haven't needed to uncoil.)



ETA: Apparently, I can upload a file, but it doesn't work the way I remember.
 
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Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
Does anybody have a circuit diagram which can be built relatively easy to verify there is an RF signal being generated by the FOB in the 300-400 MHz range. I have found some in lower ranges bit not in this range.I built a simple one using a couple of diodes, three caps and an LED but it does absolutely nothing for me.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
Hi Guys,
I bought a few of these 315/433MHz transmitter receivers and now I want to hook up the receiver portion to see if I can detect a signal from a Key FOB from my car or any other remote running on that frequency. Do I have to buy an Arduino to do this or is there a simple circuit using an LED that will allow me to see if the FOB is transmitting a signal? Thanks in advance for your help.
 

Reloadron

Joined Jan 15, 2015
7,501
Hi Guys,
I bought a few of these 315/433MHz transmitter receivers and now I want to hook up the receiver portion to see if I can detect a signal from a Key FOB from my car or any other remote running on that frequency. Do I have to buy an Arduino to do this or is there a simple circuit using an LED that will allow me to see if the FOB is transmitting a signal? Thanks in advance for your help.
The receiver has 4 pins labeled GND, VCC and two Data pins which look to be tied together. I would power up the receiver and place a scope across either data pin and ground. Place a key fob beside the receiver, push a button and see what you get. My understanding is N. American key fobs for automotive are 315 MHz. You should see the key fob data string. This is purely a guess on my part. You can also maybe take the data out and feed a opamp and drive a LED? I don't see any need for an Arduino. Looks like they use the transmitter and feed data in and then send the data to the Arduino.

Ron
 

bighand

Joined Apr 2, 2009
18
I have one of those receivers. When you have a key nearby and press one of the buttons then you get a reasonably decent square wave output which contains the key data (usually Manchester coded). However with no key signal present they pick up a LOT of random background noise, so just checking for the absence/presence of data on the output pin isn't enough.

If you put an LED on the output then it will flicker dimly the whole time. When there's a key signal being received it will light more strongly. If you just want a basic visual check to see if the key is working then that's probably enough. But if you wanted to do anything more complicated then you'd need to hook it up to a microcontroller and decode the signal.
 

Reloadron

Joined Jan 15, 2015
7,501
I have one of those receivers. When you have a key nearby and press one of the buttons then you get a reasonably decent square wave output which contains the key data (usually Manchester coded). However with no key signal present they pick up a LOT of random background noise, so just checking for the absence/presence of data on the output pin isn't enough.

If you put an LED on the output then it will flicker dimly the whole time. When there's a key signal being received it will light more strongly. If you just want a basic visual check to see if the key is working then that's probably enough. But if you wanted to do anything more complicated then you'd need to hook it up to a microcontroller and decode the signal.
That's about what I read on them, high noise.

Ron
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
I have one of those receivers. When you have a key nearby and press one of the buttons then you get a reasonably decent square wave output which contains the key data (usually Manchester coded). However with no key signal present they pick up a LOT of random background noise, so just checking for the absence/presence of data on the output pin isn't enough.

If you put an LED on the output then it will flicker dimly the whole time. When there's a key signal being received it will light more strongly. If you just want a basic visual check to see if the key is working then that's probably enough. But if you wanted to do anything more complicated then you'd need to hook it up to a microcontroller and decode the signal.
Are you placing the LED across both data terminals or to one and ground? I am just looking for a quick method with the LED but my plan is to hook it up to an Arduino in the long run.
 

Reloadron

Joined Jan 15, 2015
7,501
Are you placing the LED across both data terminals or to one and ground? I am just looking for a quick method with the LED but my plan is to hook it up to an Arduino in the long run.
The data terminals are one in the same if you look closely at the board so either terminal to common (GND). Like I mentioned earlier, also I have no idea how much current that circuit can source but bighand says it will drive the LED.

Ron
 

bighand

Joined Apr 2, 2009
18
I don't know if the receiver can source enough current to drive the LED directly. I had it connected to an input pin on a PIC micro and just ran a loop that read the pin and copied it to an output pin driving the LED. A small transistor or MOSFET would be good enough though.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
Okay I hooked it up. 5VDC power and ground and the LED to data and ground. Power on and the LED comes on and flickers a bit (noise). I tried three different key FOBS and between the 315MHZ and 433MHz, I got all of them to flash through the LED. Any thoughts on removing the random flickering when a button is not activated? You can clearly distinguish between when it is random and when the key is pressed - solid flashing as opposed to random.
 

GopherT

Joined Nov 23, 2012
8,009
Okay I hooked it up. 5VDC power and ground and the LED to data and ground. Power on and the LED comes on and flickers a bit (noise). I tried three different key FOBS and between the 315MHZ and 433MHz, I got all of them to flash through the LED. Any thoughts on removing the random flickering when a button is not activated? You can clearly distinguish between when it is random and when the key is pressed - solid flashing as opposed to random.
On a radio, you always have the carrier wave to tune into even when there is no voice/music signal to hear. In that case, you simply have silence. The problem with a key-fob system is that there is no broadcast when the button is not pressed so you have noise of any amplifed signal - especially if the board has an AGC amplifier seeking the weakest of signals (may be present, may be not). In any case, your best bet is to filter the output, not the input. I assume you have a 150 ohm resistor from the output to the LED. Put a 22 uF or 33 uF cap tied to ground and the other end between resistor and LED. This should cut off all flicker above 50 Hz to 32 Hz (respectively per 22 and 33 uF cap).
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
In any case, your best bet is to filter the output, not the input. I assume you have a 150 ohm resistor from the output to the LED. Put a 22 uF or 33 uF cap tied to ground and the other end between resistor and LED. This should cut off all flicker above 50 Hz to 32 Hz (respectively per 22 and 33 uF cap).
I have the LED placed across one of the DATA pins to ground with no resistor. The LED does not blow and it is bright when a key FOB signal is active and dimmer but flickers when there is not. I know the resistor is to drop the voltage for the LED and I did not measure the output signal voltage yet. Should I still use the same capacitor value that you mentioned in post #34?
 

GopherT

Joined Nov 23, 2012
8,009
I have the LED placed across one of the DATA pins to ground with no resistor. The LED does not blow and it is bright when a key FOB signal is active and dimmer but flickers when there is not. I know the resistor is to drop the voltage for the LED and I did not measure the output signal voltage yet. Should I still use the same capacitor value that you mentioned in post #34?
You need some resistance to make a filter. You can use a smaller resistor (half) but double the capacitance. My values were guesses of frequency. I didn't think you would be bothered (notice) by irregular flashes at 60hz and faster and I did to know what type if 'signal' you were getting from the FOB-induced LED excitation (flicker vs constant wave).

Anyhow, always a good practice to limit LED current and doubly important to have a resistor since you need a filter.
 

MrChips

Joined Oct 2, 2009
30,706
Here is a circuit that works for me:



Q1 is a high gain, low frequency transistor. The key fob has to be placed right up against the wire antenna.
 

Thread Starter

bwilliams60

Joined Nov 18, 2012
1,442
that's a nice little circuit. I could use that in class to tell when my students are on their phones :) Bet that would make them think twice. Oh, I need them to think once first :p
 
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