The objective of this simple exercise is to demonstrate the operation of solid state (or electronic) switches using electronic devices like TRANSISTORS or SCRs. There are many more electronic devices that can be used for the purpose. We have chosen these two as these are the most commonly used ones in this role, more so in the electronics circuits of hobbyists’ interest. MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is another commonly used switching device that has outclassed bipolar junction transistors in many switching applications. It is not covered here but is separately covered in the next project.

CIRCUIT DESCRIPTION

The first part of the circuit starting from extreme left and comprising of Resistors R1, R2, Switch SW1, LED-1 and Transistor Q1 illustrates the use of an NPN transistor as an ON/OFF type switch. It can be demonstrated that the transistor can be made to behave as a simple SPST type switch between its Collector and Emitter terminals. A transistor (NPN or PNP) shows an open circuit between its Collector and Emitter terminals when it is in the non-conducting or cut-off state. A transistor is non-conducting when its Base-Emitter junction is  forward biased less than 0.6 volt (Base positive with respect to Emitter in NPN and Emitter positive with respect to Base in PNP transistors). When the base-emitter junction’s forward biased voltage is greater than 0.6V, the resulting base current drives the transistor ON and a collector current equal to the base current multiplied by the current gain of the transistorflows in the collector emitter lead. When talking about transistor as a switch, if the DC base current being fed is much less than the expected collector current that would flow when the transistor is driven to the ON state (equivalent to a closed switch) divided by the current gain of the transistor, then the transistor would stay in the non-conducting state. In this part of the circuit, when switch SW1 is open, base current is zero with the result that the transistor does not conduct as indicated by the non-glowing LED-1. When SW1 is closed, base current (depending upon the value of resistance R1) flows, the transistor conducts if the collector current (as decided by resistance R2) divided by the current gain is much less (about one tenth) than the actual base current being fed. Conduction of the transistor is indicated by a glowing LED-1.

The second part consisting of Resistor R3, R4, R5, Switch SW2, LED-2 and transistor Q2 depicts the use of a PNP transistor as a switch. The concept is the same. Here also, the base current and hence the collector emitter (or more precisely the Emitter-Collector current in this case) current flows when the emitter-base junction is forward biased. And this would happen only when the switch SW2 is closed. You would notice a change in the location of the switch. This is to ensure that the emitter base voltage is less than the required forward bias of 0.6V (min) when the switch is open. Transistors are used extensively as a switch in various applications. One such application is in driving relay coils. When the transistor is switched ON, the relay coil is energised and its contacts changeover i.e. normally open contacts close and normally closed contacts open. In the circuit consisting of Resistor R6, Switch SW3, Diode D1, Relay RL-1 and the transistor Q3, the relay energised condition is indicated by a glowing LED-3. The reverse biased diode across the relay coil is to protect the transistor from any damage due to voltage spikes occuring due to sudden change in current when the transistor is switched from ON to OFF state.

SCR is another popular device used as a switch. However, it is a LATCHING device i.e. once triggered to the ON-state, it stays there even if the trigger is removed. Remember, in case of a transistor, you had to keep the switch SW1 or switch SW2 (as the case may be) closed in order to keep the transistor conducting and the LED glowing. In the SCR experiment shown, the moment you close the switch SW4, SCR conducts and the LED-4 glows. Now the LED-4 can be observed to continue to glow even if you open switch SW4. There is now only one method to bring the SCR back to the non-conducting state and that is to somehow bring its anode current below a certain minimum value which the SCR manufacturers refer to as the Holding Current. It is 10 mA for the chosen SCR. You could do that by disconnecting the anode supply (by opening switch SW5)oreven by increasing the resistance in the anode lead till the SCR goes to the OFF state indicated by LED-4 getting extinguished. Since the minimum current required to drive an LED is typically 2 mA or so, a holding current of 10 mA for the chosen SCR ensures that the LED would extinguish because of the SCR going to the non-conducting state as the anode resistance is being increased.

A relay coil placed in the anode lead of the SCR makes the circuit as a latching type relay that needs just a pulse to get energised and stay energised. This part is not shown here as it would be similar to the one shown in case of NPN transistor

Parts List	Specification
Resistors	All Resistors are carbon film or carbon composition type
R1, R3	22K ,1/4W
R2, R4, R6, R6,R9, R10	3.3K , 1/4W
R7	470 ohms, 1/4W
R8	470 ohms, 1/4W
Potentiometers
PI	10K Preset
Transistors
Q1, Q3	2N 2222
Q2	2N 2907
D1	1N4001
SCR-1	SN100 or any other 100 volts SCR
LED-1 to LED-4	Red LEDs (Other colour LEDs like green or yellow can also be used)
Relay
RL-1	6VDC relay (Fig. 2.2)
Miscellaneous
Battery	9VDC battery (Fig. 2.3)
Switches SW1 to SW5	ON/OFF switches
Solder metal multistrand wires etc

The PCB layout as seen from components’ side and components’ layout are shown in Figs. 2.4 and 2.5 respectively. The project circuit is so simple and the total number of components being used is so small that it can be conveniently assembled on a general purpose PCB if you may wish to do so.

TESTING GUIDELINES

There are four different experiments you can perform with this simple circuit. These are

(i)    Use of NPN transistor as a switch

(ii)   Use of PNP transistor as a switch

(iii)  Use of a transistor switch to drive a relay and

(iv)  Operation of an SCR as a latching switch.After you have assembled the project, before the start of every experiment, switch ON SW5 to connect the battery voltage to the circuit. For the first experiment, switch on SW1 and see if LED-1 glows. The LED should extinguish when you open SW1. Similarly, LED-2 would glow when SW2 is closed. Observe the energisation of the relay coil indicated by glowing LED-3 in the third experiment. In the fourth experiment, close switch SW4. LED-4 glows. Now open switch SW4, LED-4 continues to glow. Adjust the value of the potentiometer P1 towards increasing resistance. You would observe the LED-4 getting extinguished at a certain stage. You will notice that you have to once again close the switch SW4 and also adjust the present P1 towards reducing resistance to light the LED-4. Again, the LED-4 continues to glow when the switch SW4 is opened.

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