Published On: Thu, Aug 1st, 2019

## Logic Circuits With Classical Switching Elements

1. AND  circuit with mechanical single-pole switches

The AND circuit in figure below is an extension of the ON/OFF switch, with several single-pole manual switches connected in series.

The lamp can therefore only light up when all switches are closed or not open.

Opening any one switch, the lamp is turned off.

In this circuit, it is always queried whether all switches are closed.

If this is the case, the current can flow, energizing a lamp, coil of a contactor or relay etc.

This type of circuit is particularly suitable for safety circuits where states are required.

In series can be connected any number of various switches such as all kind of protection limit switches, pushbuttons, key pushbuttons, sensors etc.

Application:

–      Limiting movements  of various mechanical devices, drives…

–      Hedge shears,

–      Lamps with/without fuse,

–      To activate signal relays,

–      Door locking,

–      Mowing machines,

–      Paper shredders, etc.

Next figure showing one practical circuit where are used various switches connected in series on one large mining excavator.  This circuit is used for signaling and in case of some accident, fault in operation as “All stop”, or “Ready for start”.

2. OR circuit

The classical OR circuit is shown in below figure. This is an extension of the ON/OFF switch with several switches connected in parallel. The lamp or any kind of electromagnetic contactor, relay … can therefore light up or be energized when one of the switches is closed or remains.

Application:

The application of this circuit  is used to many way in the electrical control circuits, power circuit such as:

• House corridor light with buttons,
• Interior lighting in vehicles,
• Traffic lights,
• Alarm systems,
• Alarms,
• Various control circuits in devices and plants equipment…

3. Logical/Combined circuit

The logic circuit showed in the below figure is a combination of several ON/Off switches, connected in series as well as in parallel.

Application

• Industrial machines, equipment…
• In manufacturing plants,
• In vehicles,
• In computers, etc.

4. Examples

1. Example of practical applications of AND circuit

Here is AND circuit  used in hedge trimmer

Only when the circuit breaker (fuse) F1 in the table or in the house connection is closed and the plug X1 is plugged in, the first switch S1 has voltage. If now the switch S1 is pressed on the handle, the electric motor M1 can not start yet.  Only when it is ensured by pressing the switch S2 on the joint that the second hand is on the hedge trimmer, the single-phase electric motor M1 starts.

The hedge trimmer consists of two AND circuits. Take the path to the circuit breaker (fuse) box, then there are a total four AND circuits until the hedge trimmer is in operation.

2. Practical example of applications of the OR circuits

OR circuit used in staircase lighting.

The light in a staircase should be able to switched ON from each floor, but no matter where it is switched ON, all lamps (P1, P2…) must be turned ON via the contact K1 of timer relay.  The protection circuit breaker or fuse F1 must be in ON position.

This is solved with an OR circuit and a relay K1.  If one of switches – pushbuttons (S1…)  in or in front of the staircase is acutated, the relay will be energized and picks up and switches on all lamps (P1, P2, …) .

The relay K1 is a time relay, drops out (switchs off) after a set time (usually in minutes) and switches OFF all lamps.

5. Graphical symbols for logic gates

5. Problems

1. This is the negation of AND logic gate or NAND gate?

Yes, or NO.

2. This is negation of OR logic gate or NOR gate?No,

No or Yes.

3.

This is XOR logic gate.

In output is 1 if only one of input is 1?.

True or False.