Newbie needs help with triggering circuit

[cpfeiffer55]

Hi Folks, I currently am working on a triggering circuit for an application at home. I basically pulled the circuitry shown in red box from the attached image. I or-tied 5 and 6 together on the NOR gate and use ground to trigger the relay. My question is that this works well up to a certain resistance across ground and the inputs to the NOR gate however I would like to have more sensitivity. I would like the circuit to trigger if there was about 16 megohm between ground and the inputs. I was thinking of using an OP AMP but I am not certain what type or if that will work. Any help would be appreciated. Thank you in advance---cpfeiffer55

[shakalnokturn]

What type of 4001 are you using?
I would have expected 16M of pull-down enough to define a low state.
Try tying one of the inputs low permanently rather than using both simultaneously for whatever you're sensing.
The problem may be the physical setup, any length of wire between the CMOS input and resistor is going to be acting as an antenna, for all you know the gate output is changing randomly. At one point you'll also want to better define an "off" state with a high value pull-up resistor on the input(s).

[PCB.Wiz]

Hi Folks, I currently am working on a triggering circuit for an application at home. I basically pulled the circuitry shown in red box from the attached image. I or-tied 5 and 6 together on the NOR gate and use ground to trigger the relay.

You should change that to a CD4093, which has a schmitt action, to ensure the transistor is never partly turned on.

My question is that this works well up to a certain resistance across ground and the inputs to the NOR gate however I would like to have more sensitivity. I would like the circuit to trigger if there was about 16 megohm between ground and the inputs.

What pullup do you use ?
A HEF4093B  specs 0.1uA MAX pin leakage, which is > 60Mohms equivalent.
A 4093 will trigger at about 5.6V falling on 12V, so a 18 Meg pullup will give close to your 16M threshold.
Take extreme care with board leakage at those high resistances.

[SCSKITS]

What is the source that is "triggering" the circuit? Knowing this would help with getting answers that make sense.
16 megohms seems quite high and could be susceptible to noise, also any contamination on the board (or built up over time) would affect the resistance.
I agree with the other poster that a SCHMITT Trigger gate would be a better choice than the 4001.

ed

[cpfeiffer55]

The triggering input is two probes. One tied to ground and the other to the NOR gate. Looking to trigger above a threshold of conductivity of a fluid that the probes are inserted into.

[cpfeiffer55]

Thanks PCB.Wiz. I will definitely give that a try.


Update on the circuit - I just wanted to tell y'all that the 4093 worked perfectly for the triggering circuit. Thank you so much