Electronic – General directions for a timer circuit which will not use microcontroller and which can measure several hours needed


Some background info first:
In my country, price of electricity depends among other things on the time of the day when energy is used. Because of that, many people tend to turn on big energy consumers after midnight, when electricity is cheaper. One of my friends needs a new laundry washing machine, so I went browsing with her. We noticed that many new machines have a delayed start option which would be very useful because it would be possible to fill the machine and program it and it would start working by itself during cheaper time. I don't think that it would be too complicated to make a circuit which will start a washing machine after X amount of time.

More relevant part:
After examining my washing machine, I noticed that machine itself is started by a normally off pushbutton. I'm thinking that the button could be replaced by a transistor or some sort of normally off relay. Anyway that's not (yet) the problematic part for me.

The question itself:
How would I design a circuit which would wait for several hours and then "push" a button without using microcontrollers? I know that a microcontroller is the most elegant solution, but I'd rather skip collecting necessary gear for working with them at this time. Discrete logic gates, flip-flops or anything else which doesn't need a programmer is acceptable for me.

My first idea is to create or find prefabricated some sort of counter which would be connected to a slow oscillator. The oscillator would have very low frequency (say 2Hz if possible) and the counter would send signals to a device which would respond when counter reaches a certain number (say after 18 000 seconds).

I was thinking about using 74LS162/3 counters. In 74LS162's datasheet under minimum frequency it says 25MHz if I'm reading it right, so I guess that it doesn't fit my needs, but 74LS163 has 0 as minimal frequency so it looks like a logical choice to me.

As for the oscillator, the slowest crystal I could find is 32 768 KHz which is too fast for me. I thought about using capacitor and coil to make my own oscillator. Some (maybe too) basic calculations tell me that L*C needs to be 1.989*10^-2. What capacitor/coil combination would be good for that?

I didn't think too much about power supply, but I'd either use "wall-wart" adapter or check the insides of the washing machine for any available power sources.

The last part of my question are vibrations. Are there any special considerations for circuits which are going to vibrate? I was thinking of using one of those solderable protoboards for basic circuit board.

I know that a book can be written for every point, but at this moment I'm just looking for some rough ideas.

Best Answer

I know you specifically said that you did not want to use a microcontroller, but I think you should consider taking the plunge into the microcontroller world. Doing a timer like this is very easy to do on a very cheap micro since you don't need it to do anything special.

In general I have found that using a "complex" circuit of discrete components is usually plagued with minor errors in both design and construction that cause them to take a lot of time and money.

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