This circuit can be used in the car, to keep the courtesy light on after the door is closed for a preset time. Another application is give small time intervals in physics experiments or chemical or maintain lit a device panel from the touch of a pressure switch.
The time the light remains on can vary from a few seconds to several minutes, depending on both the value of the capacitor C1 used as the P1 setting.
The lamp used can be quite strong, with currents up to 3 A, and power can be sourced by voltages sources from 6 to 18 V depending only on the lamp.
In Figure 1 we have the complete diagram of the device.
The component arrangement in a terminal strip, which is indicated to the beginner is shown in Figure 2.
The transistor is a power MOSFET, may be used any suitable equivalent that supports the lamp current to be used.
The resistor is 1/8 W or higher and the pot is a lin or log type.
The capacitor should have a working voltage slightly higher than used in the supply. Its value will determine the range of obtained times.
With a 1 µF capacitor we have something around 10 seconds. With higher values we have proportionally longer times. Depending on the lamp current, the transistor need to be mounted on a heat sink.
To test the device just feed it and press S1 for an instant. The lamp should light up and remain so for a time that depends on the P1 setting. After that time it goes off automatically.
Proven operation is only to install for final use.
Q1 - IRF640 - Power MOSFET or equivalent
R1 - 100k Ω x 1/8 W - yellow, violet, yellow
P1 - 4.7 M Ω - pot
C1 - 1 to 100 µF - Electrolytic Capacitor
X1 - lamp 6 to 18 V up to 3 A - see text
S1 - Pressure Switch ON
Terminal strip, heatsink to the transistor, button to pot, wires, weld, etc.