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Can someone help me review this schematic?

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moffy:
Here is a rough schemaric of what I was talking about:  :)

redgear:

--- Quote from: mariush on July 07, 2020, 04:24:10 pm ---
ULN2003A  (darlington transistor array) or something similar to turn the relays on and off  (you can connect the microcontroller directly to this, instead of having resistor and npn transistor for each relay ... you have 7 transistor+resistor pairs inside a dip chip)
Some diodes for each relay (for voltage spike protection when relays turns off)

--- End quote ---
Thank you for suggesting this, looks great!

--- Quote ---momentary buttons with resistor on each one to limit current, put all buttons on a port and maybe set interrupt on change ... when a button is pressed an interrupt is triggered and you have your value.  (or just constantly poll the buttons which would be useful for debouncing purposes)

For extra fun, you may want to experiment with capacitive touch like you see on lots of devices .. see for example https://www.digikey.com/product-detail/en/microchip-technology/CAP1206-1-SL/CAP1206-1-SL-ND/5886099
whole category here:  https://www.digikey.com/products/en/integrated-circuits-ics/interface-sensor-capacitive-touch/560/page/2?k=capacitive%20touch&quantity=50&stock=1&ColumnSort=1000011&pkeyword=capacitive%20touch

For example

--- End quote ---
Wow, This is cool. Btw, connecting all buttons on single port won't work for me. Cos, i'm planning to include both reset and pause.


--- Quote from: moffy on July 08, 2020, 01:28:34 am ---Here is a rough schemaric of what I was talking about:  :)



--- End quote ---
Thanks a lot for the circuit:)

I m planning 0.1uF caps for C1 and C2 and a cap 470uF cap C0 before C1. And instead of transistors for relays, as mariush suggested I'm going for ULN2003A. For the voltage reg I have UA78M.

Let me know what you guys think.


Thanks again :)

moffy:
The cap C1, (C0+C1) has to be large enough that at full load i.e. max current out of 5v and all relays on, that the voltage never drops below about 8v. This is so the relays stay on, and the regulator doesn't drop out of regulation. There is a simple formula: C1 = (i*dt)/(Vmax - 8v). For 240v/50Hz, dt = 10ms.
The use of the ULN2003a is an excellent idea, just stay within the packages total current and power limitations. The built in diodes make life a bit easier also. :)

mariush:
Probably a simpler formula to use to approximate capacitance :  C  =  Current (Amps)  / [ 2 x AC Frequency x ( Vdc peak - Vdc min) ]

So if you have a 9v AC transformer, after rectification it will be 9 x 1.414 - 2 x voltage drop on diode of bridge rectifier  = 12.7- 2 x 0.8v = 11.1v peak dc voltage ... but can be higher if you use low voltage drop diodes like let's say 1n5818 (~ 0.3v at 0.1A, 0.5v at 1A) instead of a bridge rectifier.

Let's say you need 25mA for each relay, and you have 4 relays ... that's 100mA ... your micro and lcd will use another 10-20mA ... let's round it up to 125mA  and let's say you're in a 60Hz mains country

So if you want at least 10v all the time :   C  =  0.125 / [ 2 x 60 x (11v - 10v) ]  = 0.125 / 120 = 0.00104166 Farads or 1041 uF  ... so I'd use at least 1200uF 16v rated capacitor.

It would make sense to use a 12v AC transformer ... after rectification and drop of around 1.6...2v (if you use a generic cheap bridge rectifier) you'll have a peak voltage of around 15v.
Most relays should be perfectly fine with this high voltage, but if you're paranoid you can simply add a couple diodes in series to drop voltage by some amount (ex 1n400x diodes will drop 0.7v per diode) and get it closer to 12v ...

What you add in cost for diodes, you potentially save in using smaller capacitor ... C =  0.125 / 120 x (15v - 10v) = 125 / 600  = ~ 208uF

moffy:

--- Quote from: mariush on July 09, 2020, 04:43:24 am ---
So if you have a 9v AC transformer, after rectification it will be 9 x 1.414 - 2 x voltage drop on diode of bridge rectifier  = 12.7- 2 x 0.8v = 11.1v peak dc voltage ... but can be higher if you use low voltage drop diodes like let's say 1n5818 (~ 0.3v at 0.1A, 0.5v at 1A) instead of a bridge rectifier.


--- End quote ---

Nice formula. I would only say that the voltage regulation on small transformers can easily be +20%. So a no or light load your on 12v RMS can easily become 18-19v. Needs to be kept in mind, for both 5v regulator dissipation and possibly excessive voltage across the relays. Depends on a few things to do with the transformer.

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