Tutorial 2: Transistor Timer Circuit

Created on: 27 July 2012
Updated on: 12 January 2023

A transistor timer circuit for beginners in electronics that uses a single transistor. When the circuit is powered by a 9V battery, the LED switches on. A switch (or link on the breadboard) is closed to start the timer, causing the LED to switch off for a time period. After the time period is over, the LED switches on again. This is a very simple transistor timer circuit to help beginners to build breadboard circuits. The following video demonstrates the circuit.

In this tutorial, you will learn:

• How to build a slightly more complex breadboard circuit than the previous LED circuit
• More about circuit diagrams
• About transistors and capacitors

Prerequisites

You will need to have completed tutorial one where you would have learned about breadboards, resistors, LEDs and circuit diagrams.

Learn about capacitors, transistors and switches before starting this tutorial.

Transistor Timer Circuit Components

To build the transistor timer circuit, you will need a breadboard, wire links, a 9V battery, 9V battery clip, and the following components:

Qty	Part	Designator	Notes	Type
1	470 ohm resistor (yellow - violet - brown)	R1	1/4W, 5% or better	Resistors
1	1k resistor (brown - black - red)	R3	1/4W, 5% or better
1	22k resistor (red - red - orange)	R2	1/4W, 5% or better
1	470uF capacitor	C1	16V or more Also try the circuit with 1000uF and 100uF capacitors to see the timer's time period change	Capacitors
1	5mm red LED	D1		Semiconductors
1	2N2222 or PN2222	Q1	NPN transistor Can also use BC107, BC108 or BC109
1	Toggle Switch	S1	A toggle switch with single core wire soldered to its terminals for plugging into a breadboard. Alternately, a wire link can be used instead of a toggle switch.	Switches

The required parts are shown below:

Reading the Transistor Timer Circuit Diagram

The transistor timer circuit diagram for the simple transistor timer is shown below. An explanation of the parts of the circuit follows.

Reference Designators

The transistor timer circuit diagram differs from the circuit diagram of tutorial 1 in that the components now have reference designators assigned to them. A reference designator is the R1, R2, R3, D1, C1, Q1 and S1 labels that you see.

Reference designators allow easy reference to the circuit's components – for example instructions may say: plug resistor R1 into the breadboard. They are also used to identify components on a circuit board. Circuit boards will typically have reference designators silk-screened on the board next to each component. In this way it is easy to find corresponding components on the circuit diagram and the circuit board.

Power Supply

The transistor timer circuit shows that it is to be powered by 9V. The positive terminal of the power supply (our 9V battery) must be connected to the +9V wire of the circuit diagram and the negative terminal of the power supply must be connected to the wire labeled 0V.

Capacitor

The electrolytic capacitor must be connected the correct way around in the circuit – observe the capacitor's polarity – which means to connect the positive or + terminal of the capacitor where shown, and connect the minus or - terminal of the capacitor where shown in the circuit diagram.

LED

The polarity of the LED must also be observed – connect the LED the correct way around.

Resistors

As you learned in tutorial 1, resistors can be connected with either lead to 9V. Be sure to place the correct value resistor in the correct place in the circuit.

Notice that the value of R1 is 470R this is another way of writing 470Ω (or 470 ohms) and often appears in circuit diagrams where the software used to draw the circuit diagram does not have the capability to insert the ohm symbol.

Switch

We do not need to use an actual switch in the circuit, but can instead use a wire link to act as a switch by plugging it into the breadboard to close the switch or unplugging it to open the switch.

Transistor

The transistor pinout needs to be observed – the collector (c), base (b) and emitter (e) of the physical transistor must be connected as shown in the circuit diagram. Take care not to connect any of the transistor pins to the wrong part of the circuit, for example, make sure that the collector of the transistor is connected to the LED and not the base or emitter.

The mapping of the NPN transistor symbol to the physical transistor in a TO-18 metal can package is shown here:

Alternatively a PN2222 or KSP2222 transistor in a plastic TO-92 package can be used, as shown in the following image. This includes 2N2222 transistors that are packaged in a black plastic casing instead of a metal can.

Building the Transistor Timer Circuit

Insert the 470uF Electrolytic Capacitor (C1)

Bend the longer (positive) lead of the capacitor out to the left and plug it into the breadboard so that there are 5 empty holes (tie points) between the two capacitor leads. In the photo the negative lead of the capacitor is on the right. The negative lead is marked on the body of the capacitor.

Insert the 470Ω Resistor (R1)

R1 connects to the positive lead of C1, so connect it to a tie point in the same conductive strip of the breadboard. The other lead of R1 connects to the top rail of the breadboard which will be where the positive lead of the battery connects.

Insert the 22k Resistor (R2)

Connect R2 between the top rail and the negative terminal of C1.

Insert a Wire Link

Connect a wire link from the negative terminal of C1 across the middle insulating channel.

Insert the 2N2222 Transistor (Q1)

The transistor is connected so that the emitter is to the right (the lead nearest the metal tab). The base lead in the middle connects to the orange link in the photo. The collector is connected to the tie point to the left of the base.

If using the PN2222 transistor or similar black plastic part that has the same pinout, insert it with the flat face towards the capacitor. The emitter will then be on the right and the collector on the left. Connect the middle pin (base) of this transistor to the link that connects it to the negative lead of the capacitor.

Connect the Transistor Emitter to the Bottom Rail

Use a wire link to connect the emitter at the right of the transistor to the bottom rail that will connect to the negative battery terminal and is marked as 0V in the circuit.

This is what we have built so far from the transistor timer circuit schematic:

Insert the Collector Link into the Transistor Timer Circuit

Insert a link to connect the transistor collector to a tie point to the right – the purple link in the photo below.

Insert the LED

The cathode (shorter lead) connects to the transistor collector through the link.

Insert the 1k Resistor (R3)

R3 connects from the LED anode to the top rail.

Insert the "Switch" Link

A link must be connected to where the positive lead of the capacitor and a lead from R1 connect – the red wire in the photo. To close the "switch", this link will be connected to the bottom rail – leave it open for now.

Connect the Battery to the Transistor Timer Circuit

When the battery is connected to the transistor timer circuit, the LED switches on. Connect the positive wire (red) of the battery to the top rail of the breadboard and the negative wire (black) to the bottom rail.

Operating the Transistor Timer Circuit

Close the switch of the transistor timer circuit to start the timer, that is, connect the "switch" link (red link in the photo) to the bottom rail. This switches the LED off and starts the timer.

After the transistor timer circuit has finished timing or "timed out", the LED switches on again.

Try using a different value capacitor in the circuit to see the time period change. Using a 100uF capacitor will result in a very short time period. Using a 1000uF capacitor will result in a longer time period.