Eli5 : What kind of clock is in washing machines programs ?


Hello ! English is not my first language so sorry if i make mistakes. I was wondering about the kind of clock inside washing machines when you set a program for later, mine allows me to chose between a delay of 3h, 6h and 9h by pushing a switch and there’s no screen. What’s the most efficient way to build it ?

In: Engineering

Modern machines have a microcontroller inside, a tiny CPU (computer) with some basic additional functions all on the same silicon chip. A very common function is a simple real-time clock, basically a circuit that generates pulses at a constant rate attached to a counter. The CPU can read the counter to see how many pulses have happened since the counter was turned on or otherwise set back to zero.

If the timer survives the machine being turned off for a while then that probably indicates there’s a clock with battery backup running all the time. Silicon chips that do this all in a single device, just add a tiny button cell for power, are very common and cheap.

The rest is just programming the microcontroller to use the clock features to create a timer-delay. The old days of building special electronics or mechanical devices to implement complex functions like this are over.

Just a digital clock usually.

An oscillator with a known frequency and a counter circuit.

If you want to build one yourself youw can make an analogue clock from some capacitors, resistors and OP amplifiers. For example you can build an integrator that builds up current over 9 hours and 3 comparators that compare it to 1/3, 2/3 and 1 times the supply voltage. And then a reset/selector switch

I’ve seen two reply’s that explain an oscillator based clock. This is one way to solve it, but it’s too difficult to pull off efficiently and cheaply for most applications. Things that only need to get time _kinda_ right like microwave ovens or your washing machine use a different trick.
Mains power is alternating current, that switches direction (or the polarity of voltage) 50 or 60 times a second (depending on your country). The microcontroller in your device counts these oscillations as a way to keep track of time. A skilled engineer can even pull it off without a microcontroller at all, which saves a whole lot of development time and money.

The only problem with this method is, that mains power only oscillates with _kind of_ 50 or 60 Hertz, which means that it is not uncommon for these clocks to be off by a few seconds or sometimes even minutes in a matter of weeks. That’s why you might need to adjust that damn microwave clock every few months or so.