Warning!
This timer circuit uses 120 volts household mains and should only be attempted by someone that has the knowledge and skills to safely construct such a project. Otherwise personal injury and or property damage could result.

Main aquarium timer circuit board schematic.

## CD4541 Timing Calculations

To change the clock timing of the CD4541 ic. R1 and C1 set the oscillator frequency and R2 should be >10K and approximately 2 times the resistance of R1.

The CD4541 in my circuit is running at approximately 36hz and is outside of the range of the specified 1kHz to 100kHz for the RC network calculation formula from the device data sheet but seems to give a calculated frequency very close to the actual operating frequency of my timer circuit.

I have the CD4541 configured as a single transition 16 stage 65536 count timer that effectively acts as a 15 stage counter 32768 count. Below I have the time delay calculation for my aquarium timer with the oscillator running at 36 hz (cycles per second).

32768 count / 36hz = 910 seconds

910 seconds / 60 seconds = 15 minutes

Oscillator frequency calculation.
Where the frequency is between 1kHz and 100kHz.
freq. = 1 / 2.3 R1 C1

Values from my circuit plugged in.
35 = 1 / 2.3 56000 .00000022

Main circuit board drawing showing components.

## Aquarium Timer Parts List

 B1 - DF06 Bridge Rectifier C1 -  .22uf 25V Polyester Film Capacitor C2 - 1000uf 35V Electrolytic Capacitor C3 -   22uf 35V Electrolytic Capacitor D1 - 1N914 Diode D2 - 1N4001 Diode F1 - 10A (1/4" x 1 1/4") Fuse F2 - 500ma (5mm x 20mm) Fuse IC1 - CD4541 Programmable Timer IC IC2 - LM2941 Regulator LED1 Q1 -  2N2222  Transistor R1 - 56k          (All resistors 1/4 watt) R2 - 100K R3 -  15K R4, R5, R10 - 33K R6 - 560R R7 - 56K R8 - 10K R9 -  1K AC Receptacle - Kobiconn 161-0715-E Power line cord (Polarized) PCB Mount Fuse Clips Relay - T90S5D12-12   SW1 - Push button spst momentary switch T1 - Radio Shack 273-1385B Transformer (120V Primary, 12.6V Secondary)

Main circuit board. Viewing traces through top of board.

Power supply circuit board drawing showing components.

Power supply circuit board. Viewing traces through top of board.

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