555 delay-off timer with relay
Switches a load off after a set time. Press the button and a 555 wired as a monostable switches the relay on for t = 1.1·R·C, here about 11 s, then releases it: a stair light, a fan over-run or a doorbell lamp. The diode across the coil absorbs its kick when it switches off.
Build it in the labGuided build, test sheet and fault finding, free in your browser.
Parts list
- Bench power supply
- 555 timer
- 2 × Resistor
- Electrolytic capacitor
- Push button
- Film capacitor
- Relay
- Signal diode
- 12 V DC LED bulb
Test procedure
Each test has an expected value and a pass range, like a factory test sheet:
- Lamp on after a press
- On-time
How it works
What it does: Switches a load off after a set time.
Build it (guided or ready-made), then run its test procedure below. Every test has an expected value and a pass range, just like a factory test sheet.
Background
The 555 (1972) is still the most-made chip in history, and this is its most-built circuit.