The Project Asylum


Aquarium Timer






My 15 minute aquarium filter and air pump shutoff timer for fish feeding.

I used to manually switch off the filter and air pump while feeding the fish to avoid having fish food stuck in the plants and getting sucked up in the filter etc. The downside to this was that I would have to hang around to turn the power back on and sometimes I would forget about it being shut off.


Aquarium Timer.
Momentary push button switch used to trigger shutdown timer. LED indicator lit during time down period.



I had the aquarium timer idea for quite some time before I finally got around to designing and building one.

I got inspired when I came across the CD4541 programmable timer chip that somebody was asking about or suggesting to use in a issue of Nuts & Volts magazine.

One of my goals in designing the timer was that I didn't want it to use any standby power. I wanted it to use power only during the 15 minute shutdown period.


Aquarium timer rear view.
AC power in and timer unit switched power outlet.



I came very close to my power consumption goal with the exception of transformer hysteresis losses causing about 1.4 watts of power consumption at standby when the ac line voltage exceeds 120 vac.

To minimize the standby power consumption of the unit I used the normally closed contacts of the relay so the circuit only needs to be energized during the 15 minute shutdown period to hold the relay contacts open.

Another feature I spent some time on was tweaking the circuit so the timer would not activate after power line interruption.


Aquarium timer inside view.
I have safety fuses installed on the ac power input line and transformer secondary output. I chose a 273-1385B Radio Shack transformer.








Aquarium Timer main circuit board.
The main components used on the mainboard are a CD4541BCN programmable timer ic, T90S5D12-12 relay, and a LM2941 adjustable voltage regulator with a on/off input pin.

Aquarium Timer
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