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How to Make a Delay Circuit

A delay circuit is an electronic timer that, when triggered, waits a predetermined time period before acting. A delay can, for example, give you 30 seconds to key in an alarm code before setting it off, or it can act as a "snooze button," giving you 10 extra minutes of sleep. A network consisting of a resistor and capacitor together determine a time constant that sets the length of the delay. During this delay, the capacitor gradually fills up with electric charge, as a container fills with water from a narrow tube. When the capacitor becomes full, the charge triggers an LM395 power transistor, which controls external devices using up to 2 amps of current.

Things You'll Need

LM395 power transistor
Solderless breadboard
150-ohm, 5-watt resistor
4 12-inch, 22-gauge solid wires
15-volt regulated direct current (DC) power supply
100K-ohm 1/4-watt resistor
10-microfarad, 25-volt capacitor
Single-pole, single-throw momentary pushbutton switch

Instructions

- 1
  Set the LM395 transistor into the solderless breadboard, so its three leads go into adjacent columns in the board. Insert one lead of the 150-ohm resistor so it shares the column with the collector lead of the transistor. Set the resistor's other lead into a free column. Insert one end of a 12-inch wire so it connects to this column. Connect the other end of one of these wires to the positive terminal of the 15-volt power supply. Leave the other wire unconnected for now.
- 2
  Insert one end of the second and third 12-inch wires so it connects to the transistor's emitter pin. Connect its other end to the ground terminal of the 15-volt power supply. Insert the last wire into the column where the transistor's collector and the resistor wire meet. The two unconnected wire ends are the delay circuit's output.
- 3
  Connect one end of the 100K-ohm resistor to the transistor's emitter pin. Connect its other end to the transistor's base pin. Insert the negative lead of the 10-microfarad capacitor so it connects to the transistor's emitter pin. Connect its other lead to the transistor's base pin.
- 4
  Insert one pin of the switch so it connects to the transistor's base lead. Insert its other pin into the column shared by the 150-ohm resistor and positive power supply wire.

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