Simple Long Duration Timer Circuit using Two Transistors

Want to build a timer that can turn something on or off after a long time, like 2 or 3 hours?

This article shows you how to make a simple timer circuit with just two transistors.

This is a great project for anyone who wants to learn more about electronics and build something useful.

What is a Long Duration Timer:

A long duration timer is an electronic circuit designed to provide a timing function over an extended period.

This type of timer is capable of generating delays ranging from minutes to hours or even days depending on the application requirements.

Long duration timers are often used in various electronic systems and projects where a time delay is necessary for specific functions.

Circuit Working:

Parts List:

Category	Description	Quantity
Resistors	1k 1/4 W CFR	2
	10k 1/4 W CFR	1
	2.2M 1/4 W CFR	1
Capacitors	Electrolytic 1000µF 25V	1
Semiconductors	Transistor BC547	1
	Transistor BC557	1
	Diode 1N4148	1
	Diode 1N4007	1
Relays	12V Relay	1
Other	Push Button	1

The simple long duration timer circuit illustrated in the diagram above, operates as follows:

Briefly pressing the push button initiates the charging of the 1000uF capacitor.

The NPN BC547 transistor is triggered maintaining its state even after the push button is disengaged.

The 1000uF capacitor slowly discharges through the 2.2M resistor and the NPN emitter.

Activation of BC547 and BC557:

• When the BC547 is triggered it activates the PNP BC557 transistor.
• BC557 in turn, triggers the relay and the connected load.

Persistent Operation:

• The circuit continues its operation as long as the charge in the 1000uF capacitor does not fall below the BC547 cutoff threshold values.

The inclusion of the 1k and1N4148 network transforms the circuit into a highly accurate long duration timer.

This network ensures that when the transistors break the latch due to insufficient charge in the capacitor the remaining charge is forced to discharge completely through the relay coil via the resistor and diode connection.

This feature guarantees the complete depletion of the capacitor preparing the circuit for a fresh cycle.

Formulas and Calculations:

1. The below mentioned formula calculates the charge time (t_charge) of a capacitor in a simple RC charging circuit.

Let us break down the formula and calculation for the specific values from the above circuit diagram R = 2.2M and C = 1000uF:

t_charge = R × C

where,

• t_charge: This represents the time it takes for the capacitor to charge, measured in seconds (s).
• R: This represents the resistance in the circuit, measured in ohms Ω, in this case, R is 2.2 MΩ (Mega ohms)
• The value of the resistor affects how fast the capacitor charges, the charging process is slowed down by a higher resistance.
• C: This represents the capacitance, measured in farads F.
• Here, C is equal to 1000 microfarads (uF).
• The capacitors capacity to store charge is determined by its capacitance.
• It takes longer for a larger capacitance to fully charge since it can store more charge.

Calculations:

Enter the numbers in the formula:

t_charge = 2.2MΩ × 1000uF

Optional Unit Conversion:

Although it is theoretically possible to multiply Mega ohms by microfarads, the Mega ohm farad unit of measurement is not widely utilized.

It helps to convert one of the units to get a more comprehensible answer.

Because there are one million ohms in a megaohm, we may divide MΩ by 1,000,000 to convert it to F.

t_charge = 2.2MΩ × (1000uF / 1,000,000)

t_charge = 2.2Ω × 0.001F

Reduce the complexity of the equation:

t_charge = 0.0022F (Since 2.2 x 0.001 = 0.0022)

In this circuit, the capacitor will take about 2.2 seconds to charge.

Note:

The capacitor is initially assumed to be fully discharged (at 0 volts) in this formula.

The formula merely determines how long it will take the capacitor voltage to rise to roughly 63% of its ultimate value, which is the circuits voltage source.

Although it would theoretically take an unlimited amount of time for a completely charged capacitor to achieve 100% voltage, 63% is generally regarded as sufficient charging.

Leakage current in the capacitor and resistor tolerance are two examples of issues that could cause a little variation in the actual charge time.

2) Below mentioned formula can be applied to a basic RC discharging circuit in order to estimate the capacitors discharge time (t_discharge).

Let us break down the formula and calculation for the specific values from the above circuit diagram R = 2.2M and C = 1000uF:

t_discharge = R × C

A More Precise Method: The following formula can be used to calculate discharge time more precisely, particularly when working with a certain beginning voltage:

t = -RC * ln(V_discharged / V_initial)

where,

• The discharge time, t, is expressed in seconds (s).
• R is the circuits resistance, expressed in ohms Ω.
• The capacitance, expressed in farads F is denoted by C.
• The voltage you want the capacitor to discharge to, which is typically 0V in most applications is represented by the variable V_discharged.
• The term V_initial refers to the capacitor’s initial voltage prior to the start of discharge.
• The natural logarithm function, or ln is shown on the majority of scientific calculators.

Calculation:

Suppose that the capacitor in your circuit has the same initial voltage of 5V V_initial and you want to calculate the time it takes for it to discharge to 1V (V_discharged).

Your circuits values are:

R = 2.2MΩ and C = 1000uF.

Unit conversion (optional): As in the previous example, we can convert Mega ohms to Farads to make the text easier to read.

R = 2.2MΩ = 2.2Ω × (1 / 1,000,000)R = 0.0022FC = 1000uF = 0.001F

Enter the numbers in the formula: t = – (0.0022F) × C ln(1 V / 5 V)

Note: Make sure your calculator is in radian mode for the ln function.

Compute the outcome: Since time cannot be negative, the resultant value will be negative.

Since we are only concerned with the duration, we can disregard the negative sign.

The result will be around 0.0008 seconds or 0.8 milliseconds.

Remember:

This is an estimated discharge time for reaching 1V.

The actual time to reach 0V would be longer but the discharge slows down significantly as the voltage gets lower.

For better accuracy, consider using circuit simulation tools or consulting datasheets for specific capacitors, which may provide discharge time curves.

Circuit Construction:

Connect BC547

• Connect the push button to the base of the BC547 transistor.
• Connect the collector of BC547 to the positive terminal of the 1000uF capacitor.
• Connect the emitter of BC547 to the 2.2M resistor and the other end of the resistor to the negative terminal of the capacitor.

Activate BC557 and Relay:

• Connect the collector of BC547 to the base of BC557.
• Connect the emitter of BC557 to the relay coil, and the other end of the coil to the positive supply.
• Connect the collector of BC557 to the load and the negative supply.

Integrate resistor and diode:

• Connect the 1k resistor and the 1N4148 diode in series to the collector of BC547 and the positive terminal of the 1000uF capacitor.

Conclusion:

By following this guide, you can construct a highly accurate and reliable long duration timer circuit.

The formulas provided enable you to customize the timer duration based on resistor and capacitor values. Enjoy your DIY timer project!

References:

Timer circuits – difficulty analyzing