Simple Delay Timer Circuit

This post shows you how to build a timer circuit that can delay turning something on for a certain amount of time from seconds to minutes using common parts like transistors, capacitors and diodes.

This type of timer is important for many electronics to make sure things happen in the right order.

What is a delay timer circuit:

A delay timer circuit is a device which is used to introduce a delay to switch an external load such as an appliance through a relay control.

Importance of Delay Timers:

In numerous electronic applications, the incorporation of a deliberate delay, spanning seconds to minutes, emerges as a critical necessity to ensure the accurate operation of circuits.

The absence of a specified delay can lead to circuit malfunctions or potential damage.

Exploring Configurations:

The below circuit diagram illustrates the connection of transistors and passive components to achieve desired delay timing outputs.

Components:

• Transistor with a base resistor for current limiting.
• Relay serving as a collector load.
• Capacitor, a vital component, strategically placed at the end of the base resistor.

Circuit Operation:

Parts List:

A push button initiates the circuit allowing a positive voltage from the supply line to enter the base resistor, turning ON the transistor and subsequently the relay.

Simultaneously, the capacitor gets fully charged during the button depression.

Upon releasing the button the transistor persists in conduction utilizing the stored energy in the capacitor, which starts discharging through the transistor.

The relay remains activated until the capacitor fully discharges.

The capacitors value determines the time delay indicating how long the transistor stays in the conducting mode.

The base resistors value also influences the timing defining the duration for which the transistor remains ON after releasing the push button.

Limitations:

This single transistor circuit provides time delays within the range of only a few seconds.

To extend the time delay range significantly an additional transistor stage is introduced.

Advantages:

• Increased sensitivity of the circuit.
• Utilization of larger values of timing resistors enhances the time delay range.

Formulas and Calculations:

Below by connecting an RC delay circuit to a transistors base, we may determine the time delay (τ) of the circuit.

The following formula is used to compute the time constant (τ), which is a crucial component in calculating the circuit’s delay:

τ = RC

where,

• τ (tau) is the time constant in seconds (s)
• R is the resistance in ohms Ω
• C is the capacitance in farads F

Calculations:

Calculate the capacitors Farad value: To solve for the capacitance value, we must first convert it from microfarads (µF) to farads (F).

1000µF = 1000 x ^10-6 F = 0.001 F

Determine the Time Constant:
Now enter the values for the resistor and capacitor into the formula:

τ = 33,000Ω * 0.001 F = 33 seconds

Meaning

Hence, between a change at the input and the equivalent change at the output (transistors collector voltage) of the circuit, the RC delay circuit with the supplied component values will result in a delay of around 33 seconds.

Note:

This computation yields a rough figure.

Leakage currents and transistor characteristics are two examples of variables that might cause the actual delay to differ somewhat.

The time it takes for the voltage across the capacitor to approach 63% of its ultimate value is represented by the delay time constant (τ).

Depending on the circuit design, the transistors switching behavior may happen a little bit before or after this moment.

How to Build:

To build a simple delay timer circuit using a single transistor and push button follow the below mentioned steps:

Transistor Setup

• Connect the collector of the transistor to the positive supply of the PCB.
• Connect the emitter of the transistor to the negative supply ground of the PCB.

Base Resistor

• Connect one end of the base resistor to the base of the transistor.
• Connect the other end of resistor to one terminal of the push button.

Capacitor

• Connect the positive (longer) leg of the capacitor to the other terminal of the push button.
• Connect the negative (shorter) leg of the capacitor to the negative supply of the PCB.

Relay Connection

• Connect the relay coil pins between the collector of the transistor and the positive line.

Power Supply

• Connect the positive terminal of the power supply to the positive supply of the PCB.
• Connect the negative terminal of the power supply to the negative supply of the PCB.

Testing

• Press the push button momentarily.
• Observe the relay switching ON.
• Release the button and observe the relay staying activated for a duration determined by the capacitor.

Adjustments:

• Experiment with different capacitor values to adjust the time delay according to your requirements.
• You can also try different base resistor values for further customization.

Additional Note:

For an extended time delay range, consider implementing a dual transistor stage as mentioned in the second configuration.

This involves connecting another transistor in series with the first one enhancing the sensitivity of the circuit and allowing the use of larger timing resistors.

Remember to double check connections, polarities, and component values to ensure proper functionality.

Conclusion

By leveraging the principles outlined in these configurations, electronic enthusiasts can construct advanced simple delay timer circuit tailored to specific timing requirements.

Experimenting with different component values enables the customization of delay intervals showcasing the versatility of these circuits in diverse applications.

References

Delay Timer (Relay driver) for home electrical appliances