Have you ever desired a timer that after a set amount of time turns things OFF rather than ON?
Like an advanced electronic snooze button.
You may learn how to construct this type of timer circuit in this post.
The article includes:
- A list of the components you will require
- complete construction instructions.
- a brief tutorial that explains how the circuit works.
- Advice on how to modify it for personal use
This circuit is helpful for anyone who wishes to experiment with electronics.
What is a Delay ON Delay OFF Timer:
An electrical circuit or device which provides a set period of delay for turning on and off a load is called a delay ON delay OFF timer, sometimes referred to as a dual delay timer or a delay on operate delay on release timer.
This system includes of two different delay periods: the activation ON delay and the deactivation OFF delay.
These timers are often used in many kinds of industrial and control applications to arrange events manage equipment and handle activities with set delays.
Delay ON Timer Working:
Parts List:
| Category | Description | Quantity |
|---|---|---|
| Resistors | 10k 1/4 W CFR | 2 |
| 100k 1/4 W CFR | 1 | |
| 1M 1/4 W CFR | 1 | |
| Capacitors | Electrolytic 220µF 25V | 1 |
| Semiconductors | Transistor BC547 | 1 |
| Transistor BC557 | 1 | |
| Diode 1N4148 | 1 | |
| Diode 1N4007 | 1 | |
| Relays | 12V Relay | 1 |
The thing you want to turn ON after a delay the load is plugged into the relay.
When you first turn on the power electricity flows through a resistor R2.
At first the electricity can not get to a part of the circuit the base of transistor T1 because another part capacitor C2 acts like a shortcut to ground.
The electricity goes through R2 losing some power and starts to fill up C2 like a battery.
Once C2 has enough power voltage to turn on transistor T1 it flips on really quickly.
This turns on another transistor T2 which activates the relay.
At last the thing you want to turn ON the load gets power and the circuit is complete.
The delay time before the load turns ON can be adjusted by changing the resistor R2 and capacitor C2.
Another resistor R1 helps C2 empty quickly so the circuit is ready to go again soon.
There is also a diode D3 that helps block electricity at certain times in the circuit.
Formulas and Calculations:
Formula used to calculate the delay on time for timer circuits that uses a capacitor and resistor which is mentioned below:
T = 1.1 * R * C
where,
- T is the time delay in seconds
- R is the value of the timing resistor in ohms
- C is the value of the capacitor in farads
In the above circuit the timing resistor is R1 100k and the capacitor is C2 220uF.
By using the below formula one can calculate the approximate delay time of the circuit which is as follows:
T = 1.1 * 100,000 * 0.00022
T = 24.2 seconds
It is crucial to remember that this is only an estimate.
Due to several factors including component tolerances and power supply voltage the circuits actual delay time may differ slightly.
Also this formula might not work with different delay ON timer circuits and is limited to the particular circuit configuration.
Note:
The exact delay time of a circuit is measured with a stopwatch or oscilloscope.
Using online calculators or software that is designed especially for timer circuits is a further choice.
How to Build Delay ON Circuit:
To build a Simple Delay ON Delay OFF Timer Circuit follow the below mentioned steps:
- Connect one end of resistor R2 to the positive terminal of the power source 12V.
- Connect the other end of R2 to one leg of capacitor C2.
- Connect the remaining leg of C2 to the ground.
- Connect the junction of R2 and C2 to the base of transistor T1.
- Connect the emitter of T1 to the ground.
- Connect the collector of T1 to the positive terminal of the power source 12V.
- Connect the collector of T1 to the base of transistor T2.
- Connect the emitter of T2 to the ground.
- Connect the collector of T2 to the coil terminals of the relay.
- Connect the anode of diode D3 to the collector of T1.
- Connect the cathode of D3 to the base of T1.
- Connect one of the relay contacts to the load.
- Connect the other end of the load to the positive terminal of the power source 12V.
- Connect the remaining relay contact to the emitter of T2.
Adjustment:
- To set the required delay time adjust the variable resistor R2.
Final Check:
- Check that every connection is safe.
- After a set delay turn on the circuit and see the relay function.
Delay OFF Timer Circuit Working:
Parts List:
| Category | Description | Quantity |
|---|---|---|
| Resistors | 10k 1/4 W CFR | 1 |
| 100k 1/4 W CFR | 1 | |
| 1M 1/4 W CFR | 1 | |
| Capacitor | Electrolytic 220µF 25V | 1 |
| Semiconductors | Transistor BC547 | 2 |
| Diode 1N4148 | 2 | |
| Diode 1N4007 | 1 | |
| Relay | 12V Relay | 1 |
When the power is turned on electricity passes through a resistor (R2) and begins to fill a capacitor (C1) which functions similarly to a battery.
The capacitor C1 receives increasing power as it fills up.
It continues to do this until it has sufficient power to activate a tiny switch (transistor).
Another switch which is triggered by this tiny switch, activates a timer that is built using an additional resistor and capacitor.
For a set amount of time the timer allows the capacitor to recharge.
The capacitor stops the tiny switch which cuts off power to a particular relay switch when that time is up.
When it loses power the relay switch opens functioning as a bridge.
This turns the circuit off by switching off electricity to the connected device.
At last when you turn on the power again everything will be ready to start again.
Adjustment:
- By changing the resistor R2 and capacitor C1 values the delay time for switching off the load should be adjusted.
- Delay times will increase with higher R2 or C1 levels.
- The RC circuits resistor (R2) and capacitor (C1) values also have an impact on how long it takes to switch off the load.
- Depending on the desired delay adjust these values.
Formulas and Calculations:
Formula used to calculate the delay off time for timer circuits that uses a capacitor and resistor which is mentioned below:
T = 1.1 * R * C
where,
- T is the time delay in seconds
- R is the value of the timing resistor in ohms
- C is the value of the capacitor in farads
In the above circuit the timing resistor is R2 1M and the capacitor is C1 220uF.
By using the below formula one can calculate the approximate delay off time of the circuit as follows:
T = 1.1 * 1,000,000 * 0.00022
T = 242 seconds (or about 4 minutes)
Note:
This is just a tentative estimate, the actual delay time might differ based on the power supply voltage, component tolerances and other circuit features
Circuits that are similar in design to the one you have provided rely on this formula.
A stopwatch or oscilloscope is used to measure the exact delay time of your particular circuit.
Another choice is to look at online timer circuit modeling tools and calculators.
How to Build Delay OFF Circuit:
The delay OFF timer circuit you are describing is a variation of the delay ON timer but with a different relay driver configuration.
Here the relay driver BC557 is replaced by BC547 and an RC circuit is used to introduce a delay for turning the load off.
Below are the key modifications to the circuit:
Circuit Construction:
Connect R2 and C1:
- Similar to the delay ON timer connect one end of resistor R2 to the positive terminal of the power source 12V.
- Connect the other end of R2 to one leg of capacitor C1.
- Connect the remaining leg of C1 to the ground.
Connect T1:
- Connect the junction of R2 and C1 to the base of transistor T1.
- Connect the emitter of T1 to the ground.
- Connect the collector of T1 to the positive terminal of the power source 12V.
Connect T2 :
- Replace BC557 with BC547 for T2.
- Connect the collector of T1 to the base of transistor T2.
- Connect the emitter of T2 to the ground.
- Connect the collector of T2 to the coil terminals of the relay.
Connect D3:
- Similar to the delay ON timer connect the anode of diode D3 to the collector of T1.
- Connect the cathode of D3 to the base of T1.
Connect the Relay Contacts:
- Connect one of the relay contacts to the load.
- Connect the other end of the load to the positive terminal of the power source 12V.
- Connect the remaining relay contact to the collector of T2.
Power Supply:
- Connect the ground to the ground of the power source.
- Connect the positive terminal of the power source to the positive rail.
Adjustment:
- Adjust the variable resistor R2 to set the desired delay for turning the load off.
Final Check:
- Ensure all connections are secure.
- Power on the circuit and observe the relay activation and delayed switch off action.
With these modifications the circuit will now provide a delayed switch off action for the connected load controlled by the adjustment of R2 and the characteristics of the RC circuit introduced for the delay OFF function.
Conclusion:
A straightforward delay on delay off timer circuit can achieve a versatile function.
These circuits allow you to program when a device will switch off when the power is cut off (delay off) and when it will turn on when the power comes back (delay on).
They will also provide basic functions.