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Basic Transistor CircuitsAuthor
Giorgos Lazaridis
February 24, 2009

Basic transistor relay driver, actuated on HIGH input (NPN)

This circuit will drive a relay coil from a low power output, usually from an IC like 555 or a TTL/CMOS. It is used to switch high loads or loads that needs AC current to operate. The relay will be actuated when the input of the circuit goes high. The protection diode Dp is used to protect the transistor from the reverse current generated from the coil of the relay during the switch off time. The values for Rb and Qs vary accordingly. The way to calculate them is:First we calculate the load current:

IL = VS / RL

Then we calculate the transistor hFE. It must be at least 5 times the load current IL divided by the maximum output current from the Input to the base of the transistor


Now you can choose the transistor Qs. You must select it according to it's Ic that must be greater than IL and it's current gain hFE.Then you calculate the base resistor RB, If the input is taken from a component (possible an IC) that uses the same power supply as the transistor (that is Vs), then the form is:

RB = 0.2 X RL X hFE

Otherwise, if the component uses another power source (like VCC) then the form is:


The protective diode could be the 1N4001 or any general purpose diode.

An example
The output from a 74LS series TTL IC is required to operate a relay with a 160 Ohm coil. The supply voltage is 12V for the transistor and 5V for the IC. The IC can supply a maximum current of 2mA.

IL = Vs / RL => IL = 12 / 160 = 75mA

The transistor must have an hFE greater than 5 X 75 / 2 => hFE > 187.5. So we choose a transistor with hFE = 200 and IC = 100mA.
Now for the RB resistor. Because the power supplies of transistor and IC are different, we use the second formula:


With the use of some basic maths, this will produce RB = 2666.6 Ohm. You choose the closest resistor possible to this value.

And here is the circuit in action!

Basic transistor relay driver, actuated on LOW input (PNP)

This circuit will actuate the relay when the input goes to logic low. The theory of operation and the calculations are exactly as the previous circuit.

And here is the circuit in action!

Sensor switching transistor (Dark/Light activated sensor)

Dark activated sensor

Light activated sensor

The dark activated switch

The light activated switch

A transistor may be connected in a way that will switch on and off a load (RL) according to a sensor. In our example, the sensor is an LDR. The LDR is a resistor that will change it's resistance according to the light falling on the sensor. The left circuit will switch on the load when the LDR is in dark, and the right will switch on the load when light fall on the LDR.The potentiometer RS will control the sensitivity of the automation. You should consider using a potentiometer 10Kohms. Also note the resistor on the left circuit labeled RP. This resistor is used to protect the transistor when the potentiometer is switched to low values. This RP should be from 1Kohm to 22Kohms. Select according to your LDR.You should consider using low current loads using this circuit. There is a point that the transistor will not be either on or off. In this case, there is danger overheating the transistor if you have big loads like lamps and motors. In this case, you should use a second transistor connected as a driver.

And here are the circuits in action!
The dark activated relay

The dark activated relay

The Darlington pair

The Darlington pair consists of two transistors connected as the drawing. This connection have the characteristic of very high current gain. Actually, the overall gain is:

hFE = hFE1 X hFE2

This results to gain more than 10000. It requires only a tiny base current change on the input of the Darlington pair in order to switch a load.

A Darlington pair will act exactly as a single transistor only with very high current gain. Also, because there must be at least 0.7volts in both base-emitter junctions, to switch on a pair like that will need at least 1.4 Volts.

Darlington pairs are available as complete packages but you can make up your own from two transistors. The first transistor can be a low power type, but normally the second transistor will need to be high power. The maximum collector current Ic(max) for the pair is the same as Ic(max) for the second transistor.

The Darlington pair connected as touch switch

The touch switch on a breadboard

Because a Darlington pair will respond to very small current changes, you can make a touch sensor switch like seen on the schematic left. The RP is a protective resistor, about 100Kohms, in case of short-circuiting the sensor plates.

And here is the circuit in action!

Astable Multivibrator circuit

This circuit will perform an astable multivibrator circuit. It has two outputs, O1 and O2. When O1 is high, O2 is low. The frequency of oscillation is calculated by the following form:


The outputs changes as per the following timing chart:


And here is the circuit in action!

Monostable Multivibrator circuit

This circuit will perform a monostable multivibrator. When an input pulse occurs, the output goes high and remains high as per time constant of C1 and R2. While the output is high, the input pulses have no effect on the circuit. Also, If the input is triggered and kept high longer than the time constant of C1 and R2, the output will NOT stay high for longer than C1 R2 time constant. The time constant is:

T = 0.693 X C1R2

The exact timing chart is shown bellow:


And here is the circuit in action!

RS Flip Flop (RSFF)

This circuit will act exactly as an RS flip flop. The two inputs works as set and reset, and the outputs work as Q and -Q.

The timing chart for this circuit is shown below:


And here is the circuit in action!



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  • At 30 April 2015, 18:59:30 user Tom Hersh wrote:   [reply @ Tom Hersh]
    • Great tutorials! Thanks! I hope that this is a way to subscribe to your tutorials. Please sign me up!

  • At 17 February 2015, 9:45:38 user Ludmil Filkov wrote:   [reply @ Ludmil Filkov]
    • 10x for the publication!

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