A relay is an electromechanical device that lets you control motors, lamps and other high-current devices with a small current. A timer relay takes the idea a step further, keeping the relay on for a precise time period when triggered by the push of a button. You can wire a timer relay yourself for a few dollars with a 555 timer integrated circuit (IC). The 555 uses a simple resistor-capacitor (RC) network to determine a time interval, ranging from a fraction of a second to more than an hour.
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Things you need
- 8-pin dual inline IC socket
- Solder perfboard
- 30-watt soldering iron
- Electronics solder
- .1-microfarad 30-volt capacitor
- Diagonal wire cutters
- 22-gauge solid wire
- Wire strippers
- 47K-ohm ¼-watt resistors, 2
- SPST, normally-open pushbutton switch
- 2K-ohm ¼-watt resistor
- 1000-microfarad 30-volt capacitor
- 12-volt relay
- 1N914 diode
- 555 timer integrated circuit
- 15-volt regulated DC power supply
Insert the IC socket into the perfboard so its pins come out on the copper foil side. Solder pins 8 and 4 to the copper to keep the socket fixed to the board.
Place the .1 microfarad capacitor so one lead passes through a hole near pin 5 and the other near pin 1. Solder these connections and clip the excess leads. Save the clipped leads.
Trim one lead clipping to a length of about ½ inch. On the copper side of the board, solder one end to pin 6 and the other to pin 7. Trim any excess. Clip another lead to the length of about 1 inch. Solder one end to pin 4 and the other to pin 8. Trim any excess.
Insert one 47K-ohm resistor so one lead is near pin 6 and the other is near pin 8. Place the other 47K-ohm resistor so one is near pin 4 and the other is by pin 2. Solder these connections and trim the excess.
Place the pushbutton switch so one of its solder lugs is near pin 2. Use a lead clipping to connect the other solder lug to pin 1 on the copper side of the board. Solder these connections.
Insert the 2K-ohm resistor so one lead is near pin 6. Solder the connection. Insert the other lead anyplace not too near another connection. Place the positive lead of the 1,000-microfarad capacitor near this resistor lead. Solder the connection between the resistor and capacitor. Place the capacitor's negative lead near pin 1 and solder the connection. Trim all excess leads.
Insert the anode lead of the 1N914 diode near pin 1 and solder. Place its cathode lead near pin 3 and solder.
Cut four wires about 12 inches long and remove ¼ inch of insulation from the ends. Solder one end of one wire to pin 8. Connect the other end to the power supply's positive terminal. Solder one end of a second wire to pin 1. Connect the other end to the negative power supply terminal.
Solder one end of a third wire to pin 3. Solder the other end to the relay's positive coil terminal. Solder one end of the fourth wire to pin 1. Solder the other end to the negative relay coil terminal.
Place the 555 timer IC into the socket. Orient the IC so its notch matches the notch in the socket.
Turn the DC power supply on. Press the pushbutton and release. You should hear a click immediately, indicating the relay is energised, or switched on. Sixty seconds later, you should hear a second click, meaning the relay has turned off.
Tips and warnings
- Note that larger-value capacitors have a stripe on the body indicating the negative side.
- The 1N914 has a black stripe or band indicating the cathode side.
- The 1000-microfarad capacitor produces a relay "on" time of about 60 seconds. If you use a smaller-value capacitor, you will reduce the time proportionally.
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