Reversing sensors on cars can be very useful. They are now very cheap, such as this one on Ebay.
PARKING REVERSING REAR SENSORS KIT FOUR SENSOR KIT on eBay (end time 22-Feb-11 08:43:08 GMT)
However, if you have a trailer attached, they are simply a pain when reversing because they tell you useless information just when you are doing something difficult.
Now I was lucky enough to have a car that sorts it all out when I used the correct towing relay, which tells the lighting control unit which trailer bulbs are working. The car then shuts off the reversing sensors, and the car foglight, if the trailer is hooked up. It also tells me if any bulbs have failed on the trailer.
I fitted one of the Ebay reversing sensors to a friend's van, and he tows a caravan sometimes, so I put together this circuit.
The layout is for veroboard. I used surface mount resistors and a capacitor on the underside. The layout also has an LED and resistor to show when the output is on and I added a capacitor across the power rails of the op amp.
The circuit works by sensing the voltage drop in the feed to the trailer tail lamp. When the tail lamps are off, the MOSFET turns off, so that the trailer tail lamp isn't in parallel with the vehicle tail lamps. That means that the small current through the 10k resistor gives a voltage on the op amp if the bulb is missing, but nearly no voltage when the bulb is there.
The reference input of the op amp is basically the tail lamp voltage.
When the tail lamps are on, the reference voltage is slightly lower, so that a voltage drop across the transistor and the 0.1 Ω resistor will take the input below the reference.
When the tail lamps are off, the reference voltage is slightly above ground.
PARKING REVERSING REAR SENSORS KIT FOUR SENSOR KIT on eBay (end time 22-Feb-11 08:43:08 GMT)
However, if you have a trailer attached, they are simply a pain when reversing because they tell you useless information just when you are doing something difficult.
Now I was lucky enough to have a car that sorts it all out when I used the correct towing relay, which tells the lighting control unit which trailer bulbs are working. The car then shuts off the reversing sensors, and the car foglight, if the trailer is hooked up. It also tells me if any bulbs have failed on the trailer.
I fitted one of the Ebay reversing sensors to a friend's van, and he tows a caravan sometimes, so I put together this circuit.
The layout is for veroboard. I used surface mount resistors and a capacitor on the underside. The layout also has an LED and resistor to show when the output is on and I added a capacitor across the power rails of the op amp.
The circuit works by sensing the voltage drop in the feed to the trailer tail lamp. When the tail lamps are off, the MOSFET turns off, so that the trailer tail lamp isn't in parallel with the vehicle tail lamps. That means that the small current through the 10k resistor gives a voltage on the op amp if the bulb is missing, but nearly no voltage when the bulb is there.
The reference input of the op amp is basically the tail lamp voltage.
When the tail lamps are on, the reference voltage is slightly lower, so that a voltage drop across the transistor and the 0.1 Ω resistor will take the input below the reference.
When the tail lamps are off, the reference voltage is slightly above ground.
