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Can someone help me review this schematic?

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redgear:
I am a beginner and I have got few questions with the first project I am attempting. I am trying to build a reliable timer circuit with three timing options. The use case is very similar to a microwave oven i.e., the circuit should check if the doors are closed, three timing options that can be selected with the buttons, start, stop and pause options for the timer.

I first thought of using a 556 timer(restricting to only two timing options) with a relay. But I also needed a display(lcd displays, not 7 segment leds) and it was not possible to interface a display with the 556 timer.

Another option was to go with a microcontroller like arduino which is easy to work with but I'm sure about the reliability. And they also cost more.

I was searching online and found this circuit for a microwave oven controller. I'm thinking to modify this to my needs than trying to build it from scratch. I have few questions:

1) I am not able to find AT89C52, what else can I replace it with? Will any 8-bit 8051 do fine?

2) I just need to control one relay and 5 input switches. Can I just leave the other ones floating?

3) This circuit is designed to work with high power components while my total power usage will be around 80W, do I need to re-calculate for the resistors and capacitors?

4) Do I need a resistor in series with the C5 10u cap?

Thanks

EDIT: Forgot to add the image

moffy:
In your position I would be tempted to use an Arduino Nano or Uno board. Plenty of shields (what they call add on boards) LCD, relay etc. and plenty of examples as well as a simple free development and programming environment. Forget the schematic you have it will only confuse issues. The forums are very active as well.

StillTrying:
"I just need to control one relay and 5 input switches."

Plus 6 MCU outputs for sending characters to the LCD ?

If that's all you need your circuit would be much simpler than that one.

WattsThat:

--- Quote ---Another option was to go with a microcontroller like arduino which is easy to work with but I'm sure about the reliability. And they also cost more.
--- End quote ---

Did you mean to say “not sure about the reliability”? And then define the cost. They cost more than what? Various Arduino AVR based clones can purchased for less than $5 with zero effort and they are just as reliable as any other mass produced electronic device. In most circumstances, I’d say far more reliable than a one off strip board cobbled together prototype, especially so for one built by someone inexperienced.

You also have to consider programming the microprocessor. You don’t just buy an 8051 based part and go to town, what compiler? How to you get the code into the part? The edit-compile-upload cycle has a time cost and don’t overlook it. If this is your first rodeo, stick with Arduino platform, it will greatly improve you chances of success.  It’s difficult to give further advice until until we know what your experience level is, both from a hardware and software point of view.

janoc:

--- Quote from: redgear on July 05, 2020, 08:52:26 am ---
1) I am not able to find AT89C52, what else can I replace it with? Will any 8-bit 8051 do fine?


--- End quote ---

No, depending on version it may not have any ROM, it may be old EPROM type, it may have only 4k flash instead of 8k, it could be one time programmable ...

Try AT89S52 instead of 89C52, that version is still commonly available and can be programmed in-system (unlike the 'C' series which needs an external programmer that you insert the chip into).

But for a new project I would rather use an AVR (= Arduino) than 8051 series, it is *much* easier to program for.

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