Automotive Auxiliary Light Timer

About: Bit of a background in various electrical and mechanical fields, obscure sense of humour and typically willing to help...

The automotive circuit described here allows a 12V 14A current to remain on for a specified time after the ignition is turned off. In this specific case it is near 20 minutes. The auxiliary source is a resistive load like a mobile tool battery charger or light.

The Auxiliary load is turned on with the ignition and remains on for the timed length after the ignition is turned off.

The board files and schematic are included, I used Eagle to make these.

The Circuit boards were produced by a local to Calgary company. Yes they are AWESOME!!!!

Step 1: Specifications

This circuit was to allow a timed auxiliary signal that
could be switched with an ignition signal but was isolated from the battery source. This was designed for remote location use and it had to be small. Components may need to be changed out in the field when necessary if spares were not available, this was to replace an existing “black Box” that had become NLA(no longer available).

I identified a location in the electrical system that was suitable and had easy access. There were existing plugs that could be separated, there were 6 connections but only 3 were used, the other 3 were simple pass pass through wiring.

Field soldering surface mount components is problematic to say the least. I chose to go with through hole. The other problem was size, I ended up with a size that had to fit inside a standard Hammond 1551KBK enclosure.

The proof of concept circuit produced had several problems. The first and probably most important was the “ignition” signal had to stay on for 20 minutes past the vehicle power down. So, the hand built board shown here was a total failure.

Step 2: Circuit Description and Operation

The client liked the size so that had to be kept.

I decided to start over and redefine everything…

Needs, simple timing, isolation, 10A current switching, seamless integration into the existing circuitry.

Ideally the timing could be accomplished with a Trip5 timer and the current switched through a pair of MOSFETs, more on this in a bit…

Looking at the schematic that I built, Pins 1 is 12VDC from the vehicle battery and goes to the Pin 1 load side , Pin 2 is Trigger or Ignition and does not carry through the board and Pin 3 is Ground on both ports of the board. On the load port, the AUX is powered when the ignition is on and for 20 minutes after vehicle shutdown. These 4 are the only ones that matter for general use.

There are 2 troubleshooting LEDs, the red one is for 12V power and the green one indicates the state of the switche auxiliary load.

The trigger is a high pulse from the ignition circuit of the vehicle, It stays high as long as the vehicle is running. This ignition signal is used to switch a Darlington transistor, this in turn produces a LOW signal which is fed into the timer IC

This part of the circuit is basic. The timing is a function of the Resistor R1 and Capacitor C1 on the schematic. The timer portion is calculated at 1.1 * R1(M) * C1(uF) in this case gives 1.1 * 33* 33 = 1197.9 seconds or 19.965 minutes. Close enough to 20 minutes to not cause any problems. Use the data sheet for the timer IC to make any changes to the basic circuit.

The timer chip outputs a high signal until the cycle is complete, then it goes low.

This output signal is used through an opto-isolator to completely separate the ignition signal and the auxiliary power.

The isolator is a TLP351 for us in heavy switching environments. (I had several on hand from a previous project) produces an inverting output that is used to trigger a MOSFET pair.

With the current switching part, I needed to change an inverted signal.

The first MOSFET IRFI540NPBF is a N-channel and absolute pure overkill (100V 20A) but I had several and it was required to change the now inverted timing signal to the correct state for triggering the output current to the load.

At the time of writing this MOSFET has a direct substitute part number FDPF3860T.

The Second MOSFET IRF9530NPBF P-channel is the real workhorse and switches the 12-14 Volts DC at up to 14A, the specification originally called for 8A continuous, I usually like to double it but again the circuit works as specified.

All the files included have not been altered from the original. This includes the missing trace from the positive side of the smoothing capacitor to pin8 of the timer IC. I also forgot to include cutouts for the case fasteners.

If you are wondering I installed 38 of these in vehicles and one busy season completed without issues, now for the winter...

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    2 Discussions


    4 weeks ago

    But what are they being used to power? You forgot to say. Its a strange requirement to be left on 20 minutes after ignition off but switched on all the time the car is going .

    1 reply

    Reply 4 weeks ago

    It was being used to charge mobile radios in graders. I believe they were also using it for GPS asset tracking. I use one in my truck to power aux offroad and cargo lights so they stay on while the ignition is off.