Simple Delay Timer Circuit

This article explains how to use basic components like transistors, capacitors and diodes to create a timer circuit that may delay turning things on for a specific amount of time ranging from seconds to minutes.

Many circuits require this kind of timer to ensure that events occur in the correct sequence.

What is a delay timer circuit:

A device that uses a relay control to generate a delay while switching an external load such an appliance is called a delay timer circuit.

Importance of Delay Timers:

In many electrical applications making use of an intended delay ranging from seconds to minutes becomes important for improving circuit performance.

The lack of a specified delay may result in damage or circuit faults.

Exploring Configurations:

Transistors and passive parts are connected to provide the necessary delay timing outputs as shown in the circuit diagram below.

Components:

• Transistor that limits current by using a base resistor.
• The relay is acting as a collector load.
• At the end of the base resistor lies an essential component called a capacitor.

Circuit Working:

Parts List:

The circuit begins by pressing a push button which permits a positive supply line voltage to enter the base resistor and turn on the transistor and relay.

The capacitor charges completely at the same time as the button is pressed.

When the button is released the transistor continues to conduct by using the capacitors stored energy which causes the transistor to begin discharging.

The relay remains off until the capacitor has completely discharged.

The value of the capacitor controls the time delay which shows how long the transistor remains in the conducting state.

The timing is also affected by the value of the base resistor which controls how long the transistor stays ON when the push button is released.

Limitations:

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

To extend the time delay range particularly an additional transistor stage is included.

Advantages:

• By increasing the sensitivity of the circuit.
• Using of larger values of timing resistors increases the time delay range.

Formulas and Calculations:

Following are the formulas for Simple Delay Timer Circuit:

By connecting an RC delay circuit to a transistors base we can figure out the time delay (τ) of the circuit.

The following formula is used to calculate the time constant (τ) which is a important component in calculating the circuits 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 one must first convert it from microfarads (µF) to farads (F).

1000µF = 1000 x ^10-6 F = 0.001 F

Calculate the Time Constant:

Now enter the values for the resistor and capacitor into the formula:

τ = 33,000Ω * 0.001 F = 33 seconds

Meaning:

With the above component values the RC delay circuit will thus produce a delay of about 33 seconds between a change at the input and the equivalent change at the circuits output (transistors collector voltage).

Note:

The result of this calculation is an estimated number.

Variables like as transistor characteristics and leakage currents might result in a little variation in the actual delay.

The delay time constant (τ) is the amount of time it takes for the voltage across the capacitor to get close to 63% of its final value.

The transistors switching behavior might occur somewhat before or after this point depending on the circuit design.

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.
• The emitter of the transistor connect to the negative supply ground of the PCB.

Base Resistor

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

Capacitor:

• Connect the positive (longer) terminal of the capacitor to the other terminal of the push button.
• The negative (shorter) terminal of the capacitor should be connected to the negative supply of the PCB.

Relay Connection:

• The relay coil pins should be connected between the transistors collector and the positive line.

Power Supply:

• The power supplies positive terminal should be connected to the PCBs positive supply.
• Connect the power supplies negative connector to the PCBs negative supply.

Testing:

• For some time press the push button.
• Watch as the relay turns on.
• Release the button and watch as the relay remains active for a period of time that is set by the capacitor.

Modifications:

• Try adjusting the value of the capacitor to modify the time delay suit what you want.
• For more modification one can also experiment with different base resistor values.

Additional Note:

Consider using a dual transistor stage as shown in the second design for a longer time delay range.

By putting a second transistor in series with the first the circuits sensitivity is increased and bigger timing resistors can be used.

To ensure correct functioning remember to verify connections, polarities and component values twice..

Conclusion:

The ideas presented in these configurations can be used by electrical hobbyists to build advanced simple delay timer circuits that are modified to meet particular timing needs.

By experimenting with various component values delay intervals can be altered showing the circuits versatility to a wide range of applications.

References:

Delay Timer (Relay driver) for home electrical appliances