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Basic timer II

The timer is based on the well-known display driver LM 3915, which has a logarithmic scale. This may be seen strange, but is not because a simple RC network is used for time measurements.  The voltage across this network, R6-P1-C1, has, while C1 is being charged, an exponential character ( U = Uref-t/RC ). When this is applied to IC1 which takes its logarithm, the time is shown linearly on the display. The values of P1 and R6 have been chosen to enable the timer is rest (C1 is discharged) when S2 is closed. Resistor R1 limits the peak value of discharge current from C1 to an acceptable value for the switch contacts.

Resistor R5 compensates the leakage currents of the electrolytic capacitors to obviate the risk of D10 going out.

If pin 9 of IC1 is left open, the display is in shown by only one LED lighting. After a reset, D1 will light first and then, sequentially, all the other LEDs.

The total current drawn by the timer is 20 mA, so that battery supply is possible. When pin 9 is connected to pin 3 (+ve supply line), the display is in the bar mode. After a reset, all LEDs will light, after which they will go out, in turn, starting with D1. In this mode, battery supply is not recommended.

Basic timer II Schematic diagram

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