Adjustable Timer Circuit
Variable delay timer using 555 IC in monostable mode with relay output
Project Overview
This adjustable timer circuit uses the 555 timer IC in monostable mode to create variable time delays from seconds to minutes. Perfect for controlling lights, fans, or other devices that need to run for a specific duration.
What You'll Learn:
- 555 timer monostable operation and timing calculations
- Relay control circuits and flyback diode protection
- Transistor switching and driver circuits
- Variable timing control using potentiometers
- Power supply considerations for relay circuits
Ideal for bathroom fans, stairway lights, automatic watering systems, and garage door timers.
Required Components
Everything needed for this intermediate project
Step-by-Step Instructions
Insert the 8-pin IC socket into the breadboard or PCB
Connect power supply: pin 8 to +12V, pin 4 to ground
Wire the timing network: Connect 1MΩ potentiometer between +12V and pin 7
Connect pin 7 to pin 6 (threshold) and pin 2 (trigger)
Connect timing capacitor C1 (100µF) from pin 6/2 to ground
Add power supply decoupling capacitor C2 (0.01µF) across power pins
Connect momentary push button from pin 2 to ground for manual trigger
Wire transistor driver circuit: pin 3 to base through 1kΩ resistor
Connect relay coil between collector and +12V with flyback diode
Connect LED indicator in parallel with relay coil for visual feedback
Insert 555 IC and test with different potentiometer settings
Always use a flyback diode across the relay coil to protect against voltage spikes when the relay switches off.
Circuit Theory & Operation
The 555 timer operates in monostable mode, where it produces a single output pulse when triggered. The duration of this pulse is determined by the RC timing network.
Timing Calculation:
T = 1.1 × R × C
Where R is the potentiometer resistance (0-1MΩ) and C is the timing capacitor (100µF)
Range: 0 to 110 seconds (adjustable from 0 to nearly 2 minutes)
Operation Sequence:
- Press trigger button to start timer
- 555 output goes HIGH, turning on transistor
- Relay energizes and contacts close
- LED indicator lights up
- After timeout period, everything turns off
Key Features:
- Single trigger activation
- Visual LED feedback
- High current relay output
- Adjustable timing range
- Automatic reset after timeout
Troubleshooting Guide
Timer doesn't start
Check trigger connection and ensure momentary button makes good contact
Timer runs continuously
Verify timing capacitor polarity and check for proper reset connections
Timing period incorrect
Adjust potentiometer or replace timing capacitor with different value
Relay doesn't activate
Check transistor connections and ensure adequate supply voltage
LED always on
Verify correct wiring of output stage and check for short circuits
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View ProjectApplications & Modifications
Practical Applications:
- Bathroom exhaust fan timer (5-10 minutes)
- Stairway light delay timer (1-2 minutes)
- Workshop equipment timeout (adjustable)
- Garden sprinkler system timer
- Security light delay circuit
Circuit Modifications:
- Add a reset button to stop timer early
- Use different capacitor values for different time ranges
- Add multiple timing ranges with a selector switch
- Include a retriggerable input for extended operation
- Add digital display to show remaining time
Success Criteria: Circuit triggers reliably, timing is adjustable across full range, and relay operates properly with visual feedback.