The Planted Tank Forum - View Single Post - DIY pH Contorller

g8wayg8r · 01-29-2006, 09:57 PM

I've been tinkering with DIY controllers for the last two years and I've built a lot of prototypes and followed a lot of plans somewhat unsuccessfully at times and this is the best I can offer for my effort to date.

The meter portion of the controller is a modification of plans provided by National Semiconductor. It uses an ultra low current op-amp (LMC6001). Their plan uses +/- 5VDC so the pH is read out in tenths of a volt, e.g. 0.7 volts is a pH of 7. I didn't like that so I desigend the circuit to run off of +/- 12 VDC. I had to make adjustment to the resistor values so that 1 volt is 1 pH unit and I've provided the equations I used iterate resistior values in a spreadsheet to get a workable combination. Rather than using a precision voltage control diode (I burned my last one up), I decided to simply use a red LED, which seems to work.

The output goes into a comparator (not an op-amp). The set point for the pH is provided by a precision voltage regulator that is adjustable. When I set it at 6.6 volts, it stay put and may only drift 10 or 20 milivolts. Hysteresis is provided by a potentiometer and a resistor in series to give me 0.02 to about 0.12 volts offset. I can control a range in pH of 0.04 with no problem.

The output for the comparator turns the solenoid on through a series of transistors. The first two are general purpose and for reasons I can't explain are needed. The first one triggers an LED to show that the comparator is calling for CO2 and it in turn triggers a Darlington pair that activates the solenoid, which runs on 12 VDC. I've got a simple snubber (a capacitor and and a resistor) to soften the activation. Normally you see circuits like this with a diode to protect the transistors. Since the solenoid is on the emitter side of the transistor I believe I don't have to be as careful but there is a LED attached in the same manner that not only verifies that the solenoid is activated, it provides a path to divert any voltage spikes.

The whole unit is powered by the +/- 12 VDC from a old computer power supply.

01-29-2006, 09:57 PM	#1 (permalink)
g8wayg8r Planted Tank Enthusiast (View User Tanks) PTrader: (0/0%) Join Date: Dec 2003 Location: St Charles, MO Posts: 516	DIY pH Contorller - New and Improved I've been tinkering with DIY controllers for the last two years and I've built a lot of prototypes and followed a lot of plans somewhat unsuccessfully at times and this is the best I can offer for my effort to date. The meter portion of the controller is a modification of plans provided by National Semiconductor. It uses an ultra low current op-amp (LMC6001). Their plan uses +/- 5VDC so the pH is read out in tenths of a volt, e.g. 0.7 volts is a pH of 7. I didn't like that so I desigend the circuit to run off of +/- 12 VDC. I had to make adjustment to the resistor values so that 1 volt is 1 pH unit and I've provided the equations I used iterate resistior values in a spreadsheet to get a workable combination. Rather than using a precision voltage control diode (I burned my last one up), I decided to simply use a red LED, which seems to work. The output goes into a comparator (not an op-amp). The set point for the pH is provided by a precision voltage regulator that is adjustable. When I set it at 6.6 volts, it stay put and may only drift 10 or 20 milivolts. Hysteresis is provided by a potentiometer and a resistor in series to give me 0.02 to about 0.12 volts offset. I can control a range in pH of 0.04 with no problem. The output for the comparator turns the solenoid on through a series of transistors. The first two are general purpose and for reasons I can't explain are needed. The first one triggers an LED to show that the comparator is calling for CO2 and it in turn triggers a Darlington pair that activates the solenoid, which runs on 12 VDC. I've got a simple snubber (a capacitor and and a resistor) to soften the activation. Normally you see circuits like this with a diode to protect the transistors. Since the solenoid is on the emitter side of the transistor I believe I don't have to be as careful but there is a LED attached in the same manner that not only verifies that the solenoid is activated, it provides a path to divert any voltage spikes. The whole unit is powered by the +/- 12 VDC from a old computer power supply. __________________ Andrew, Eheim Club Member #24 This message is always under construction: 75-gallon tank; 2, Eheim 2026 filters; Tek Light with 4, 54W T5s (6000K) ; Sand on top of 4:1 sand:clay mixture; Milwaukee CO2 controller; PlantGuild vortex CO2 reactor; pH = 6.6, kH=70mg/l, GH=120mg/l; EI; Flourish excel on 50% weekly water change: AGA Member. Last edited by g8wayg8r; 06-09-2007 at 05:33 PM.