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R1______________4K7 1/4W Resistor
R2,R3___________1K 1/4W Resistors
C1____________100΅F 25V Electrolytic
Capacitor (See Notes)
D1___________1N4002 100V 1A Diode
Q1____________BC547 45V 100mA NPN
Transistor
Q2____________BC327 45V 800mA PNP
Transistor
P1_____________SPST Pushbutton
RL1___________Relay with SPDT 10A min.
switch
Coil Voltage 12V. Coil resistance
150-600 Ohms
This device is a simple timer, allowing
to keep on the headlights of your vehicle for about
1min. and 30sec., e.g. when accessing some dark place,
without the necessity of coming back to switch-off the
lights.
Pushing on P1 allows C1 charging to full 12V battery
supply. Therefore Q1 is driven hard-on, driving in turn
Q2 and its Relay load. The headlights are thus activated
by means of the Relay contact wired in parallel to the
vehicle's headlights switch. RL1 remains activated until
C1 is almost fully discharged, i.e. when its voltage
falls below about 0.7V.
The timing delay of the circuit depends by C1 and R1
values and was set to about 1min. and 30sec.
In practice, due to electrolytic capacitors wide
tolerance value, this delay will vary from about 1min.
and 30sec. to 1min. and 50sec.
An interesting variation is to use the inside lamp as a
command source for the timer. In this way, when the door
is opened C1 is charged, but it will start to discharge
only when the door will be closed, substituting
pushbutton operation.
To enable the circuit acting in this way, simply connect
the cathode of a 1N4002 diode to R1-C1 junction and the
anode to the "live" lead of the inside lamp.
This lead can be singled-out using a voltmeter, as it is
the lead where a 12V voltage can be measured in respect
to the vehicle frame when the lamp is on.
100΅F = 1'30" to 1'50"
47΅F = 0'45" to 1'05"
220΅F = 3'15" to 4'15"
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