Switch-On Circuit Diagrams 40 Young Engineers Alarmic Zhou 1995

In this post we discuss the making of simple delay timers using very ordinary components like transistors, capacitors and diodes. All these circuits will produce delay ON or delay OFF time intervals at the output for a predetermined period, from a few seconds to many minutes. All the designs are fully adjustable.

Importance of Delay Timers

In many electronic circuit applications a delay of a few seconds or minutes becomes a crucial requirement for ensuring correct operation of the circuit. Without the specified delay the circuit could malfunction or even get damaged.

Let's analyze the various configurations in details.

You may also want to read about IC 555 based delay timers. Recommended for you!

Using a Single Transistor and Push Button

The first circuit diagram shows how a transistors and a few other passive components may be connected for acquiring the intended delay timing outputs.

The transistor has been provided with the usual base resistor for the current limiting functions.

A LED which is used here just indication purposes behaves like the collector load of the circuit.

A capacitor, which is the crucial part of the circuit gets the specific position in the circuit, we can see that it's been placed at the other end of the base resistor and not directly to the base of the transistor.

A push button is used to initiate the circuit.

On depressing the button momentarily, a positive voltage from the supply line enters the base resistor and switches ON the transistor and subsequently the LED.

However in the course of the above action, the capacitor also gets charged fully.

On releasing the push button, though the power to the base gets disconnected, the transistor continues to conduct with the aid of the stored energy in the capacitor which now starts discharging its stored charge via the transistor.

The LED also stays switched ON until the capacitor gets fully discharged.

Te value of the capacitor determines the time delay or for how long the transistor stays in the conducting mode.

Along with the capacitor, the value of the base resistor also plays an important role in determining the timing for which the transistor remains switched ON after the push button is released.

However the circuit using just one transistor will be able to produce time delays which may range only for a few seconds.

By adding one more transistor stage (next figure) the above time delay range can be increased significantly.

The addition of another transistor stage increases the sensitivity of the circuit, which enables the use of larger values of the timing resistor thereby enhancing the time delay range of the circuit.

PCB Design

Video Demonstration

Using a Triac:

The following image shows how the above delay timer circuit may be integrated with a triac and used for toggling a mains AC operated load

The above could be further modified with a self contained power transformerless power supply as shown below:

simple compact transistorized timer circuit

Without a Push-Button

If the above design is intended to be used without a push button, the same may be implemented as indicated in the following diagram:

The above delay OFF effect without a push button can be further improved by using two NPN transistor, and by using the capacitor across base/ground of the left NPN

The following circuit shows how the associated push button may be rendered inactive as soon as it's pressed and while the delay timer is in the activated state.

During this time any further pressing of the push button has no impact on the timer as long as the output is active or until the timer has finished its delay operation.

Delay from an External Trigger

Problem asked by Mr. Glen (one of the dedicated readers of this blog):

I have a situation where I have a pulse of 12V that lasts about 4 seconds (from a rotary switch being turned by a slow motor) but I only want about half a second pulse (to trigger a mechanical bell/chime).

Is there any way to take a long pulse into a circuit and send a much shorter pulse out?

The solution to the above problem is provided in the following schematic:

Two Step Sequential Timer

The above circuit can be modified to produce a two step sequential delay generator. This circuit was requested by one of the avid readers of this blog, Mr.Marco.

A simple delay OFF alarm circuit is shown in the following diagram.

The circuit was requested by Dmats.

The following circuit was requested by Fastshack3

Delay Timer with Relay

"I am looking to build a circuit that would control an output relay. This would be done in 12V and the sequence will be initiated by a manual switch.

I will need an adjustable time delay (possibly displayed time) after the switch is released, then the output would go on for an adjustable time (also possibly displayed) before shutting off.

The sequence would not restart until the button was pressed and released again.

The time after the button release would be from 250 milliseconds to 5 seconds. The "on" time for the output to turn on the relay would be from 500 milliseconds to 30 seconds. Let me know if you can offer any insight. Thanks!"

So far we have learned how to make simple delay OFF timers now let us see how we can build a simple delay ON timer circuit which allows the connected load at the output to be switched ON with some predetermined delay after power switch ON.

The explained circuit can be used for all applications which calls for an initial delay ON feature for the connected load after the mains power is switched ON.

Delay ON Timer Circuit Working Details

The shown diagram is pretty straightforward yet provides the necessary actions very impressively, moreover the delay period is variable making the set up extremely useful for the proposed applications.

The functioning can be understood with the following points:

Assuming the load which requires the delay ON action being connected across the relay contacts, when power is switched ON, the 12V DC passes via R2 but is unable to reach the base of T1 because initially, C2 acts as a short across ground.

The voltage thus passes through R2, gets dropped to relevant limits and starts charging C2.

Once C2 charges up to a level which develops a potential of 0.3 to 0.6V (+ zener voltage) at the base of T1, T1 is instantly switched ON, toggling T2, and the relay subsequently....finally the load gets switched ON too.

The above process induces the required delay for switching ON the load.

The delay period may be set by appropriately selecting the values of R2 and C2.

R1 ensures that C2 quickly discharges through it so that the circuit attains the stand by position as soon as possible.

D3 blocks the charge from reaching the base of T1.

Parts List

R1 = 1o0K (Resistor for Discharging C2 when circuit is switched OFF))
R2 = 330K (Timing Resistor)
R3= 10K
R4 = 10K
D1 = 3V zener diode (Optional, could be replaced with a wire link)
D2 = 1N4007
D3 = 1N4148
T1 = BC547
T2 = BC557
C2 = 33uF/25V (Timing Capacitor)
Relay = SPDT, 12V/400 Ohms

PCB Design

Application Note

Let's learn how the above delay ON timer circuit becomes applicable for solving the following presented issue by one of the keen followers of this blog, Mr. Nishant.

Circuit Problem:

Hello Sir,

I have a 1KVA automatic voltage stabilizer.It has one defect that when it is switched on, very high voltage is outputted for about 1.5s (therefore cfls and bulb got fused frequently) after that the voltage becomes OK.

I have opened the stabilizer it consist of an auto-transformer,4 24V relay each relay connected to a separate circuit(each consisting of

10K preset,BC547,zener diode,BDX53BFP npn darlington pair transistor IC,220uF/63v capacitor,100uF/40V capacitor ,4 diodes and some resistors).

These circuits are powered by a step down transformer and output of these circuit are taken across corresponding 100uF/40V capacitor and fed to corresponding relay.What to do in order to tackle the problem.please help me.Hand drawn circuit diagram is attached.

Solving the Circuit Problem

The problem in the above circuit might be due to two reasons: one of the relays is switching ON momentarily connecting the wrong contacts with the output, or one of the responsible relays is settling down with the correct voltages a little while after power switch ON.

Since there are more than one relay, tracing out the fault and correcting it can be a bit tedious......the circuit of a delay ON timer explained in the above article could be actually very effective for the discussed purpose.

The connections are rather simple.

Using a 7812 IC, the delay timer can be powered from the existing 24V supply of the stabilizer.
Next, the delay relay N/O contacts may be wired in series with the stabilizer output socket wiring.

The above wiring would instantly take care of the issues as now the output would switch after some time during power witch ONs, allowing enough time for the internal relays to settle down with the correct voltages across their output contacts.

Feedback from Mr. Bill

Hi Swagatam,

I stumbled across your page doing research on the web to make my delay more consistent.Some back ground information first.

I am a bracket drag racer and launch the car on first sight of the 3rd amber bulb as the christmas tree is coming down.

I use a transbrake switch that is depressed to lock the automatic transmission in forward and reverse at the same time.

This allows you to rev up the engine to build power for launch. When the button is released the transmission comes out of reverse and moves the car forward under high rpm.

This is like popping the clutch on a manual transmission car, anyway my car reacts to quickly and the result is a redlight, leaving to early, and you lose the race.

In dragracing your reaction time on the launch is everything and it is a game of hundreths-thousanths with the big boys, so I have put the transbrake switch on a relay and put a 1100uf cap combo across the relay to delay its release.

Because of the car electronics I don't believe there is a precise voltage charging this cap every time I activate this circuit and precision is key so I bought a power stabilizer off of Ebay that takes 8-15 volts in and gives a consistent 12volts out.

This turned my season around but i believe this circuit could be made to be more precise and to vary the delay time in an easier way rather than swap cap combos.

Also should I run a diode in front of the relay, not currently because all that is there is the on off switch- where will the current go? I am not an electrical engineer by any means but do have some knowledge from trouble shooting high end audio for many years.

Would love your thoughts- thankyou

Bill Korecky

Analyzing and Solving the Circuit

Hi Bill,

I have attached the schematic of an adjustable delay circuit, please check it out. You can use it for the mentioned purpose.

The 100K preset can used and adjusted for acquiring precise short delay periods as per your specifications.

However, please note that, the supply voltage will need to be minimum 11V, for the 12V relay to operate correctly, if this is not fulfilled then the circuit might malfunction.

Regards.

one transistor relay delay ON timer circuit

Simple 5 to 20 Minute Delay Timer

The following section discusses a simple 5 to 20 minute delay timer circuit for a specific industrial application.

The idea was requested by Mr. Jonathan.

Technical Requirements

While trying to figure out a solution to my problem on google, I came across your above posting.

I'm trying to figure out how to build a better Sous Vide controller. The main problem is that my water bath has a very high hysteresis, and when heating from colder temperatures will overshoot about 7 degrees from the temperature at which power is terminated.

It is also very well insulated, with a gap between the inner and outer vessel which makes it act like a thermos jar, because of this it takes a very long time to decline from any excess temperature. My PID controller has an SSR control output and a relay alarm output.

The alarm can be programmed as a below limit alarm with an offset from the set-point. I can use a five volt supply I already have for my circulation motor to run through the alarm relay and drive the same SSR the control output is driving.

To be on the safe side and protect the PID controller I'll add a diode to both the alarm voltage and the control voltage to prevent one output from feeding back into the other.

I'll then set the alarm to stay on until the temperature rises above the set-point minus 7 degrees. This will allow the PID tuning to be adjusted without having to account for the initial temperature ramp-up.

Because I know that last few degrees will be achieved without any power input, I'd really like a way to delay any recognition of the control signal for about five minutes after the alarm shuts off, as it will still be calling for heat.

This is the part I've yet to figure out the circuitry for. I'm thinking of a normally closed relay in series with the control output, which is held open by the alarm signal.

When the alarm signal is terminated, I need a delay on the order of five minutes before the relay returns to its 'off' normally closed state.

I would appreciate help with the delayed off portion of the relay circuit. I like the simplicity of the initial designs on the page, but I get the impression they wouldn't handle anywhere near five minutes.

Thank you,

Jonathan Lundquist

The Circuit Design

The following circuit design of a simple 5 to 20 minute delay timer circuit can be suitably applied for the above specified application.

The circuit employs the IC4049 for the required NOT gates which are configured as voltage comparators.

The 5 gates in parallel form the sensing section and provides the required time delay trigger to the subsequent buffer and the relay driver stages.

The control input is acquired from the alarm output as indicated in the above description. This input becomes the switching voltage for the proposed timer circuit.

On receiving this trigger, the input of the 5 NOT gates are initially held at logic zero because the capacitor grounds the initial trigger via the 2m2 pot.

Depending upon the 2m2 setting, the capacitor starts charging up and the moment the voltage across the capacitor reaches a recognizable value, the NOT gates revert their output to logic low, which is translated as a logic high at the output of the right single NOT gate.

This instantly triggers the connected transistor and the relay for the required delay output across the relay contacts.

The 2M2 pot may be adjusted for determining the required delays.