HCH-1000-002 Circuit Design Help.

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Yeah there's a lot of work here. My mouse battery's are going dead. LOL As for the power supply, it's confirmed. i use that set up on my thermostats and motor controls. It read stead no matter what you throw at it. I just finished the order so i'll let you know how it turns out.
 
Good luck. Remember: improvise when you need to do so. Don't be too rigid on the values of the parameters.

Edit: You might wanna try the capacitance meter too in the 2917 datasheet.. but the minimum capacitance is too high for detection, I'd just want to know how it turns out for you. Just an afterthought.
 
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OK, that didn't work. The voltage output was in the low mVs. Range was to small also. However i built this circuit on the bread board and it works like a charm. The voltages @ the VCC are 5.0v @ 0% humidity and 10v @ 100%. So now i need help figuring out the resistor values to get me down to this range from the 12v source for the LM358 comparator.
 
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let me see.. you don't need R16 & 17 for voltage division, because the voltage detected by the inv. input of your comparator is actually purely just the ratio of the resistance between 1E-1S and 1S-1A.

Try that first. I'm very glad your output for v2f worked out as planned. Nice work!
 

So I'm not comparing voltages between the output of the VCC and the 1s? How about the r14 is it necessary? Thanks
 
R14 is good practice, I'd leave it there. The voltage through the inv input goes according to the voltage divider principle: (Resistance 1E-1S divided by the total potentiometer resistance) x (Vcc-Gnd). Also, 1A goes to ground directly.

For inverted results, just swap the inv and non-inv input with each other.
 

I don't know what value to use for the R14.
 
I don't know what value to use for the R14.
Click to expand...
It's not really critical, something between 10K and 33K should be OK. Is there any reason why your using a BC547 ? Also In the power supply block of your schematic the 1N4002 diode (D1) and the 2 1N4148 diodes after the 12 V. regulator are not necessary.
 

i use the bc547 because i have plenty of them. I use them in other projects so it keeps my reorder sheet small.

Thanks for the input. I would like to limit the number of components on the pcb. I tried to build a pcb last night on a 3" x 4" board. What a rats nest.
 
since you're only switching the relay, you don't need to be particular about it. As long as the relay switches on, go crazy. A simple way to test the relay is to take 3V, series with a 1K resistor, apply +ve to the base, -ve to GND, the relay should switch on with a 'tick' sound. When this works, the same should work for your main circuit.

No need for precision in this case, but if you're wondering, the switching current of a BC547 is only 0.5mA, shouldn't be a problem. Like I said, just use any resistor values until the relays clicks.

**broken link removed**
 
cool thanks for the info.
Here is the altered design. i had to swap the #6 & #5 pins on the comparator and i had to add the r14 for hysteresis. The circuit was switching on/off with a 5% variance. Now with the r14 switches at 1%. There isn't any relay chatter. I just guessed the r value. It seems to work good.
The only problem is when i first turn on the circuit. It stays on at any pot setting for about three minutes. Then turns off and switches properly. My assumption is that it's the caps charging. I might need to incorporate a polarized cap in the mix to fix this.
 
i use the bc547 because i have plenty of them. I use them in other projects so it keeps my reorder sheet small.
Click to expand...
I see, I was just curious because those type of transistors are not at all common in the US.
 
I'm not sure what happened when you put R14.. but now that is just an inverting amplifier, with a 1000 gain. But since it works albeit not perfectly.. well I suggest you replace 1N4148 with an IN4001 or other series. IN4148 are high speed data/signal diodes, the IN4001s are suited for much higher ratings. That might show something.

...and to improve the threshold of the op-amp output, try using higher values of the R14. It might help the pull-up effect of the output.
 
OK, i got everything functioning properly and mounted on a PCB. I have only one issue to resolve. The LED on the board begins to light up slowly before the relay switches. I would rather it be either on or off without anything in-between. Is this the R18 value? if so what value do i need to swap it with? If it's not the R18 then what is it and how do i resolve the issue? Thanks for all of your help thus far. I am so excited to have a functioning humidistat.
 
That's strange. Probably there are drift currents driving the base, providing some allowance for the transistor to be in forward biased although only a small amount of current.. it'll turn on the LED. I say this'll be remedied by putting your 1N4148 (now's the time to use it!!) in series into the base of your transistor.

One more thing: Did you wash your PCB already? There might be residual flux which has some resistance throughout your board. Just wash it using soap to remove the flux grime, dry it and test again.

Congrats!
 
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That's interesting about the washing of the board. I'll give it a shot first. then i'll add in the n14148. Thanks again.
 
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