The post explains a simple automatic darkness activated switch for car head lamps and the DRLs, initiated by the ignition triggers. The circuit helps to save battery power and prevents unnecessary wastage of energy. The idea was requested by Mr. Vladnemir.

Technical Specifications


I would like to build switch for driving car lights.
I want to use HID pair (2x35W) and DRL 2x8(LED 1W per diode)
Everything must be sensitive on ambient light and working just with key in the ignition (key sense +6V).

Thank you 

Designing the Circuit


The requested design of a ignition triggered and darkness activated dual head lamp circuit can be easily made using the ubiquitous IC 555.

As shown in the figure below, the entire circuit consists of two exactly identical stages comprises of two IC 555 configured as comparators.

The LDRs are used for sensing ambient light conditions and their deteriorating intensities.R1 on both the circuits are adjusted such that the respective ICs attain a particular light detection thresholds depending upon the user preference.

For example the left R1 may be adjusted such that the relay just triggers when sun just begins to set, while the right hand side R1 may be set such that its relay gets activated only when its completely dark.

As long as the above levels do not reach, pin2 of the ICs are allowed to receive a ground potential which keeps the corresponding pin3s at high, thereby inhibiting the relay activations.

As soon as the respective thresholds are crossed, pin2 goes high via R1, consequently pin3 becomes low allowing the relay to get activated and illuminate the relevant lamps as suggested by the requester.

Pin4 which is the reset input of the ICs are integrated to the ignition positive of the vehicle so that the circuit responds with the above actions only when the car or the vehicle is being used and not while its parked or not in use.

Circuit Diagram




The above diagram indicates how the proposed darkness activated car head lamp and DRL circuit can be built using a couple IC 555 stages

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