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How to Use Time Delay Circuit to Slow Down a Contact

By itself, a momentary switch closes an electrical circuit as long as you keep your finger on the button. When you release it, the switch breaks the circuit. For some applications, you may want a time delay before the circuit actually closes a contact, as with a home alarm system that gives you time to key in an entry code. In this circuit, one 555 timer integrated circuit, or IC, creates a 1-second delay. It then flips a second 555 timer on for 1 second. A pushbutton contact starts the first timer. After the delay, the second timer outputs a pulse you can use to turn on a relay or transistor.

Things You'll Need

Single pole, single throw momentary pushbutton switch
Assorted 22-gauge jumper wires, 2 to 10 inches long
Soldering iron
Electronics solder
Solderless breadboard
2 555 timer ICs
9 volt battery clip
2 1N914 diodes
100K-ohm, 1/4-watt resistor
2 22-microfarad 35-volt capacitors
3 2K-ohm, 1/4-watt resistors
4 .1-microfarad 35-volt capacitors
3 47K-ohm, 1/4-watt resistors
9 volt battery
Multimeter

Instructions

- 1
  Prepare the momentary pushbutton switch by soldering one end of a 2-inch wire to each of its two terminals.
- 2
  Align the two 555 timer ICs on the breadboard so the pins straddle the horizontal slot in the middle of the board, and the orientation markings on the ICs point in the same direction. Push them into the board carefully, checking that all pins go in straight.
- 3
  Insert the red 9-volt battery clip wire so it connects to pin 8 of the first IC. Connect pin 8 of the first IC to pin 8 of the second with a jumper wire. Connect pins 4 and 8 of the first IC with a jumper wire, and do the same with the second IC. Connect pins 6 and 7 of the first IC with a jumper wire, and make the same connection on the second IC with another jumper wire.
- 4
  Insert the black battery clip wire so it connects to pin 1 of the first IC. Connect pin 1 of the first IC with pin 1 of the second IC with a jumper wire.
- 5
  Set the anode lead of one 1N914 diode so it connects to pin 8 of the first IC. Connect the cathode lead to pin 2. Connect the 100K-ohm resistor between pins 8 and 2 of the first IC. Set one lead of a 47K-ohm resistor so it connects to pin 8 of the first IC, and connect its other lead to pin 7.
- 6
  Connect the negative lead of one 22-microfarad capacitor to pin 1 of the first IC. Connect its positive lead to a free column on the board. Insert one lead of a 2K-ohm resistor into this column. Connect its other lead to pin 7. In the same manner, connect the second 22-microfarad capacitor and 2K-ohm resistor to the second IC.
- 7
  Insert one lead of a 2K-ohm resistor so it connects to pin 8 of the first IC. Insert its other lead into an unused column on the breadboard. Set one lead of a .1-microfarad capacitor so it shares this column. Connect its other lead to pin 2 of the first IC. Insert one wire soldered to the pushbutton switch so it shares the connection between the 2K-ohm resistor and .1-microfarad capacitor. Connect its other lead to the first IC̵7;s pin 1.
- 8
  Connect a .1 microfarad capacitor between pin 3 of the first IC and pin 2 of the second IC. Set the anode lead of the second 1N914 diode so it connects to the second IC̵7;s pin 8. Connect its cathode lead to the second IC̵7;s pin 2. Connect a 47K-ohm resistor between pins 2 and 8 of the second IC. Insert a 47K-ohm resistor so its leads connect to the second IC̵7;s pins 7 and 8.
- 9
  Connect a .1 microfarad capacitor between pins 1 and 5 of the first IC. Do the same for the second IC.
- 10
  Snap a fresh 9-volt battery into the clip. Insert one end of a jumper wire so it connects to pin 3 of the second IC.
- 11
  Turn the multimeter on and set its selector knob to read DC volts. Touch the red multimeter probe tip to the wire coming from the second IC̵7;s pin 3. Touch the black probe tip to the second IC̵7;s pin 1. Press the pushbutton and release it. If all the connections are correct, you should see the voltage rise after a delay of 1 second, then it will drop after another second.

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