Car Light Bulb

Quick increase-decrease delay

One-shot operation

Circuit diagram:

Car Light Bulb


R1_____________470K  1/4W Resistor
R2_____________100K  1/4W Resistor
R3_____________220K  1/4W Resistor
R4_____________100K  1/4W Resistor (See Notes)
R5_____________220R  1/4W Resistor (See Notes)

C1,C2___________10µF  25V Electrolytic capacitors
C3,C4___________47nF  63V Polyester capacitors

Q1________STP36NF06L  60V 30A N-Channel MosFet or similar

IC1____________4093 Quad 2 input Schmitt NAND Gate IC
IC2___________LM358 Low power Dual Op Amp

P1_____________SPST Pushbutton Switch, any type suitable to the vehicle

LP1__Car Light Bulb 12V 55W

Battery_________12V Vehicle battery


This circuit was designed on request: i.e. a bulb that gradually turns on (1 sec. period), stays on for 1-2 sec. and then gradually turns off in about the same lapse of time.
Due to the high current involved (almost 5A for a 55W bulb) a PWM circuit was devised, in order to drive efficiently the MosFet output device. One of the two op amps contained into IC2 compares two different triangular waves present at its inputs and generates a Pulse Width Modulated rectangular wave.
At pin #2 of this IC a 110Hz triangular wave is present, obtained from a square wave oscillator formed by IC1A, R3 and C3 and followed by the integration network R4-C4.
A single cycle triangular wave with a period of about 6 sec is present at pin #3 of the same IC and is generated by pushing on P1. C1 and R1 set the delay of the first half-period, i.e. the delay during which the output pin (#10) of IC1C will be high. During this half period of about 3 sec. C2 charges through R2 and then, when pin #10 goes low, discharges in about the same lapse of time.