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LM122 Timer IC

LM122 Timer Introduction.
The LM122 Timer is an integrated circuit used as a timer, a pulse generator, tone generator and many other circuit functions, all from one universal IC function. Also refer to the links to the 555 timer description below.

LM122 Timer Voltage Supply.
The supply voltage [Vcc] may be set to any level between 4.5V and 40 volts. The Ground pin [GND] only has to be more negative than the Vcc line and does not have to be zero volts. A dual supply may be used to power the LM122 timer. However if a dual supply is used the output emitter needs to go to the more negative voltage and not taken to ground.

The Vref pin is an output, although its shown on the left side of the circuit. The point of the Vref line is to provide a stable reference to the RC charging circuit. Vref will remain stable even as the supply voltage might fluctuate, providing a fixed voltage for the capacitor to charge to.

LM122 Analog Timer IC Internal Block Diagram
LM122 Block Diagram

LM122 Operational Description.

Output Drive.
Output Floating Transistor. Because the output is both an open collector and open emitter, the output may either be taken off the emitter or the collector. Also because the collector is open the line may be taken to a different level than Vcc. As with any open collector line, just pull the collector pin to a supply voltage via a resistor and connect the load to the collector pin. When using the emitter as an output, pull the collector to a voltage and place the load in series with the emitter and ground [or more negative supply].

Reset Control.
The reset transistor reads the output of the comparator. Once the transistor turns on, the timing capacitor is shorted and provided a path to discharge, which would than start the timing process again. The Vadj line is the positive input to the comparator. As the block diagram shows, the line is already tied to a fixed voltage, but the pin allows an external connection.

External Control.
This section is TBD

Timing Equations: [not yet confirmed]
Charge Time; t1 = 0.693*(R1 + R2)*C1
Discharge Time; t2 = 0.693*R2*C1
Total period; T = t1 + t2 = 0.693*(R1 + 2*R2)*C1
Oscillation frequency; f = 1/T = 1.44/[(R1 + 2*R2)*C]
Duty cycle = D = R2/(R1 + 2*R2)

Editor note; I'm not sure if the LM122 is still in production

Generic LM122 Packages: 14-pin SOIC, and 14-pin DIP.

555 Timer Circuit Descriptions:
555 Monostable Circuit
555 Astable Circuit
555 50% Duty Cycle Circuit, Astable
555 Individual Pulse Width, Astable
555 Decreasing Frequency, Astable
555 Capacitor Adjustment, Astable

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