Introduction

I needed a timer for the UV TL's which I use to expose my printed circuit boards.
The TL's are used only occasionally for approximately 3 minutes to expose the printed circuit boards.
It is clear that a timer with very low off-state power is required here......

I designed a simple timer that switches off it's own mains power supply when the load is switched off.
The on time is set by the RC-time of a resistor and an elcap in combination by a schmitt trigger CMOS gate.
Practical on times of 10 seconds to about 30 minutes can be archived.
The timer concept is not very accurate, but once set, the on-time of the circuit proved to be quite stable.

I made two versions: a simple timer and a "de luxe" version with a beep signal just before switchoff

Version 1: simple timer

The on time of the timer is set by the one shot built with R2/R3 C3 and IC2A: after pressing pushbutton S1
C3 will be charged slowly through R3/R2; as long as the voltage of C3 is below the schmitt trigger trip level,
the output of IC2A will be high and the the relay is switched on.
Altogether the relay will switch on immediately after pressing pushbutton S1: The relay contact will take over the mains current.
The load can be switched by another relay contact (if available) or can be put in parallel with the 12V transformer
primary mains terminals
When the voltage on C3 reaches the schmitt trigger trip level, the output of IC2A will become low.
The relay will switch off the mains power supply of the timer and the load.
The timer is reset by R1 which will discharge C3 fast through D1.

Simple timer schematic:

Version 2: "de luxe" timer

The "de luxe" schematic makes use of all four schmitt trigger nand's of the HEF4093 IC

The on time is set by the one shot built with R2/R3 C3 and IC2A: when the pushbutton is depressed
C3 will be charged slowly through R3/R2; as long as the voltage of C3 is below the schmitt trigger trip level,
the output of IC2A will be high.
C6 will be charged fast through D4: the inputs of IC2D become high, the output of IC2D will become low and T1 starts conducting.
Altogether the relay will switch on almost immediately after pressing S1.

Switchon:
During the on-time, C4 is not charged because the output of IC2A is high and R4 is also connected to +8V
Because the inputs of IC2B are both high during on-time, the output of IC2B is low and the oscillator
built with IC2C, R5 and C5 is off: no beep-sound from the buzzer

Switchoff:
When the voltage on C3 reaches the schmitt trigger trip level, the output of IC2A will become low.
The inputs of IC2B will be low for a short time, until C4 is charged to the trip level. During this time
the oscillator built with IC2C will be switched on: the buzzer will beep for about one second.

Because the output of IC2A becomes low, C6 is also discharged. this will take a little longer than
the time the buzzer is made active (the buzzer timing is set with C4/R4).
When C6 is discharged below the trip level, T1 stops conducting and the relay switches off
the load and the mains supply.


R1 will discharge C3 and C4 fast through D1 and D2: because of this, the timer is reset in a few seconds.

The buzzer is a piezo disc as used in watches and greeting cards. An ordinary speaker will also work
but a DC blocking capacitor (100nF) must be placed in series with the speaker.

"De luxe" timer schematic:

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