Op Amp output noise Problem

[Lighty]

Anyone that can help me on using a Logic gate for controlling the above? Please help

 

[audioguru]

Sorry, I don't know what you are doing and I don't know what you want.

The ph is a number that tells how acidic or alkaline is a liquid. But you want to switch something when the ph is above or below a "set" value??

 

[Lighty]

Hi Audioguru,

Going to be using it to control my fish tank system.

Ch1 will be connected to a Cacium Recator where the pH will be set at about 6.55. when it reaches that the solenoid valve will turn of the CO2, thereby stop it from going any lower.

This solution from the reactor will be pumped into the tank slowly, BUT people have had problems with setting it up and have offen pump too quickly, and therefore reducing the pH in the Tank to dangerious levels.

So and probe on ch2 will be placed in the tank to monitor and if neccesary overriding (switching off) the valve controlled by Ch1.

thx

 

[ericgibbs]

Hi


THE PROBLEM

Because the input signal changes very slowly, when its at the threshold point of the opamp (against the Ref Voltage) it start to form a "square Wave" output and causing the relay to "vibrate", and not acting as a "clean" switch.

Attched is a basic schematic of the comparator and relay driver.

Thanks in advance.

hi,
Look at this mod to your circuit.
Try a 100K feedback.

The diode is to protect the transistor.

 

[Lighty]

hi,
Look at this mod to your circuit.
Try a 100K feedback.

The diode is to protect the transistor.

Thanks Eric, I've actually using a 2M pot for feedback, found even 1M was given to much hysteresis, also have put 2 diodes in, one across the relay and the other across the transistor, found it recommended on the data sheet.

the problem now is the logic gate. Are there any AND logic ICs with only 2 inputs that can handle Vcc at 15V? (need to keep the design small) or should I just use 2 transistors in series?

Also another problem is, when the feed from the opamp is LOW Voltage is about -15V, could I just use a diode to only supply positive voltage to the Logic Gate?

 

[ericgibbs]

hi,
Where are the inputs to the AND gate going to be connected and what will the AND gate output be driving.??

 

[Lighty]

inputs are from the opamps, which are configured as "schmitt triggers" if I understand correctly.

Output will drive a 12V relay.

 

[ericgibbs]

inputs are from the opamps, which are configured as "schmitt triggers" if I understand correctly.

Output will drive a 12V relay.

hi,
As the cct is not showing any values, I can only assume the voltage divider gives a reduced logic level.

Whats the value of the divider resistors and the comp supply voltage.?

 

[Lighty]

supply will be + - 15V. originally used the resistors as a voltage divider to drive a transistor, but would like it to drive a logic gate dirrectly.

basically what i'm wanting to do is outputs from the opamps into the Logic IC (AND) to drive a relay. so both ouputs from the opamps must be high to turn on the relay.

 

[ericgibbs]

supply will be + - 15V. originally used the resistors as a voltage divider to drive a transistor, but would like it to drive a logic gate dirrectly.

basically what i'm wanting to do is outputs from the opamps into the Logic IC (AND) to drive a relay. so both ouputs from the opamps must be high to turn on the relay.

I know that you want to level shift the Comp outputs to logic levels, just use the circuit I posted with a resistor in each diode input.

Both inputs will have to be high in order to switch the relay.

Do you follow OK.?

EDIT:
Look at this image.

 

[Lighty]

not 100%,

put resistors in place of the diodes or just place in series with the diode?

Could I not use a circiut like the attached one?

thanks for your help.

 

[Lighty]

I know that you want to level shift the Comp outputs to logic levels, just use the circuit I posted with a resistor in each diode input.

Both inputs will have to be high in order to switch the relay.

Do you follow OK.?

EDIT:
Look at this image.

I see you editted it, so its that easy?

 

[Lighty]

Hi,

I've attached a pic of the final schematic, If you could please check and see if i've got it right, i've broken it down into coloured block to help, the "connection" points will go to a display board where it will be displayed on 3 different Voltmeters.

The threshold op-amps have a hysteresis resisters for chatter free switching.

The outputs from the pH circuits go to a AND logic to act as a safe gaurd, ensuring the tank pH is above a set limit for the relay to opperate.

Component values not shown.

PLEASE advise if I have made any error with the schematic.