Ralay technology has been replaced by plc, expecially for industrial applications.

A plc program can be written by using some different languages, among which ladder diagram. That language seems like an electromechanical circuit schematic: its main symbols are virtual relays.

 

Than we can say the old technology is still now used.

 

 

  

 

 

 

The following pdf file contains a plc program that works for a 4 floors elevator. I used step7 software (Italian version).

I simulated it with s7200 cpu.

 

S0 (I0.0) = count magnetic switch (up and down); speed exchange at middle floors.

S1 (I0.1) = mechanical switch at the first floor (speed exchange and program condition reset).

S2 (I0.2) = mechanical switch at the top floor.

S3 (I0.3) = stop magnetic switch (up and down).

P1...P4 (I0.4...I0.7) = driving buttons on the floors.

C1...C4 (I0.4...I0.7) = driving buttons in the car.

FTC (I1.0) = photocell, door open button.

 

S (Q0.0) = up power relay

D (Q0.1) = down power relay

GV (Q0.2) = high speed power relay

PV (Q0.3) = low speed power relay

AP (Q0.4) = door open power relay

CP (Q0.5) = door close power relay

RO (Q0.6) = floors driving delay (NT)

 

 

 

 

 

 

This simple circuit can be utilized to drive a monostable relay, using a single button switch.

When I press the button IC output (pin 3) assumes the high level, but Q1 transistor switches off. So when I release it the relay is energized, and the exchange switch closes on 5 and 9.

When I press the button again IC output assumes the low level, but Q3 transistor switches on: when I release it the circuit goes to the start condition.