Programmable Digital Countdown Timer for Food Dehydrator Design

[thedoug]

I'm looking to build a food dehydrator and have the specs all worked out except for the electronics part of it. The "timer" I'm looking to build for it is basically a countdown timer I'd like to control with a trimmer or pot of some sort to set the amount of drying time in hours on the four 7 segment displays and have a start button. When the timer counts down all the way the dehydrator turns off. This seems like it should be a fairly easy circuit to build but I can't find anything online schematic wise but RC type timers and digital clocks and that's not what I'm looking to do. Basically the circuit would work just like a microwave circuit/display does...no idea how to control the BDC with a rheostat or if that's totally in the wrong direction etc....any help would be greatly appreciated!

 

[alec_t]

Perhaps a simple mod of a count-down kitchen timer would be easiest/cheapest? Just wire up to the 'start' button and the beeper.

 

[thedoug]

that might be the easier way to do it but not the cheapest. I'm going to put these dehydrators out on the market so the circuitry needs to be my own. I guess my question really is how to get the rotary dial to encode to the PIC as a keypad would. Maybe a small range of voltages that range from 0-5V in increments...dunno, have to think about this more I guess...

 

[cowboybob]

There's Cana-Kits: https://www.canakit.com/2-hour-digital-countdown-timer-with-led-display-kit-ck247-uk247.html

I've built and used this one but it may not have the timimg interval your looking for.

Also: **broken link removed**

This has a longer (up to 24hr) countdown interval.

Your wanting a digital readout is adding a good bit to the price. But even still, these are going to be cheaper than doing it yourself (unless cost is no object).

There are also wall switch replacement timers by Tork.

 

[cowboybob]

thedoug,

Ignore my previous post which I was writing as you posted yours (I'm a very slow typist and my spell checker isn't working).

Didn't know you were planning on selling these and that low cost was a factor.

You say you don''t want a digital clock but you want to emulate the digital clock on a microwave . Understand that a digital readout adds considerably to the cost.

What sort of timing range(s) are you considering? What sort of accuracy (minutes, seconds, hours)? My experience with food dehdyrators is that it sometimes takes days for stuff to dry out (tomatoe slices, for example).

There's no penalty for using someone else's design or product to operate your's, just so long as you pay for it.

 

[dougy83]

that might be the easier way to do it but not the cheapest. I'm going to put these dehydrators out on the market so the circuitry needs to be my own. I guess my question really is how to get the rotary dial to encode to the PIC as a keypad would. Maybe a small range of voltages that range from 0-5V in increments...dunno, have to think about this more I guess...

If you want to use a pot then you can use an ADC pin on the PIC. If you wanted to set the timer in units of 10 minutes, a four-digit display could have 1000 different values for time duration. If your circuit board is designed properly, the 10-bit ADC may actually give you 1024 usable positions, though the value will potentially jump around. If you had e.g. 12 bits, you could ignore certain codes to reduce the jump-around/instability.

You could alternately use a rotary encoder, which will not have the limit of 1024 values. As they can come with an built-in switch, you could have a coarse- and fine-adjustment feature - e.g. coarse mode (30-minutes / click) by default, and fine mode (1-minute / click) when the knob is pushed while turned. Rotary encoders may cost a bit more than pots however. Ten units can be bought for under $5 incl. postage from ebay.

 

[thedoug]

Awesome suggestions! I was thinking about the times needed for the unit and really don't think that I'll need a rotary switch like I was thinking in the first place but maybe 2 push button switches, one for days and one for hours. The PIC might be a little much as well as I really don't see a need for it if I just put in a countdown timer circuit with a 60Hz crystal in it. That way I negate the need for a PLC programming unit, of which I do not have. Someone should invent some kind of Allen-Bradley type of software that you can plug in a USB to your computer and some connector to use on your breadboard and do the programming that way. Seems like it would take some of the bulk away from the design table.

Anyway, on to this project. If I use just one 7 segment display for the days and two 7 segment displays for the hours I think I can get away with just a push button to set them and a reset button to reset them and then a counter circuit without the PIC. What would I need besides a latch and maybe some 555 circuits in series?

 

[dougy83]

Anyway, on to this project. If I use just one 7 segment display for the days and two 7 segment displays for the hours I think I can get away with just a push button to set them and a reset button to reset them and then a counter circuit without the PIC. What would I need besides a latch and maybe some 555 circuits in series?

You'd need three BCD up/down counters, three 7-segment display decoders/drivers, and at least one oscillator. You can get counters combined with display decoders in the same chip; perhaps even one with up/down counting.

Alternately you could use one PIC.

 

[alec_t]

Won't you also need to monitor door closure etc and have some alarm/reset features? To do all the timing plus handling user input, display and the extra features points to use of a PIC or other micro. It would be cheaper and simpler overall than using separate logic and counting ICs, which would need quite a complex pcb.