... 6 producing an automatic restart. These clock pulses can drive a flashing LED, a loudspeaker, a transistor, an IC, etc. A 741 operational amplifier or op-amp is used as a comparator or amplifier with one output and two inputs. A voltage at the '+' input causes a positive output (source) if it is more than the '-' input and a negative output (sink) if it is less than. The voltage gain is 100 000 and the output current is 10 mA. As gain is very high resistors are used to produce negative feedback and give a lower but predictable and constant voltage gain. Motors and Relays 1. A relay uses induction to isolate circuits. Other inductors include motors and solenoids. A protective diode is used to prevent back emf. 2. A two-pole relay is used for motor reverse control and latched (memorised) switching. Formula 1. For resistors in series the effect is increased: R total = R1 ...

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