Making snail sensor independent of voltage

Thread Starter

Paramecium

Joined Jun 2, 2021
19
Hi !
I’m new to electronics, but now I’ve made a circuit for a sensitive touch sensor to be used with snails. The goal is to have the sensor as sensitive as possible without making false alarms, and at the same time drawing as little current as possible. The motor (M1) should be activated for about 5 seconds. That time can be adjusted by the capasitor (C1) and the resistor (R4).

The circuit I have made seems to work fine on all accounts, except that the activation time vary a lot depending on the state of the battery, due to the battery voltage. That is the problem. I have also made a regular timing circuit using the TS555, but I can’t make that one sensitive enough.

Any suggestions would be highly appreciated!
Bilde_2021-06-02_173729.png
 

Alec_t

Joined Sep 17, 2013
12,210
Welcome to AAC!
Perhaps using the opamp output to trigger a 555 configured as a monostable would do what you want.
With only a 3V supply you're pushing the limits of the specified opamp and MOSFET.
 

Thread Starter

Paramecium

Joined Jun 2, 2021
19
Welcome to AAC!
Perhaps using the opamp output to trigger a 555 configured as a monostable would do what you want.
With only a 3V supply you're pushing the limits of the specified opamp and MOSFET.
Thanks!

The monostable circuits I have seen works by grounding the trigger pin, and thus the timing is set by the capasitor charging time rather than discharging. That makes the timing independent of the battery voltage, which is good. Problem is how to make the op-amp trigger this without making it too complicated.
 

Ian0

Joined Aug 7, 2020
3,268
Just as an aside, a 0Ω (single black band resistor) just measured 17mΩ using a 4-wire measurement. So... maybe we need some superconducting resistors...
Fancy that - I never realised that you could buy resistors with less resistance than a zero-ohm link! My current project has a 10mΩ resistor for current sensing.

You need SPICE if you really want to get confused between milli and Mega! "M" is still "milli" in SPICE.
 

Ian0

Joined Aug 7, 2020
3,268
Hi !
I’m new to electronics, but now I’ve made a circuit for a sensitive touch sensor to be used with snails. The goal is to have the sensor as sensitive as possible without making false alarms, and at the same time drawing as little current as possible. The motor (M1) should be activated for about 5 seconds. That time can be adjusted by the capasitor (C1) and the resistor (R4).

The circuit I have made seems to work fine on all accounts, except that the activation time vary a lot depending on the state of the battery, due to the battery voltage. That is the problem. I have also made a regular timing circuit using the TS555, but I can’t make that one sensitive enough.

Any suggestions would be highly appreciated!
View attachment 240263
First, get all your design parameters together - what is the resistance of a snail?
 

Thread Starter

Paramecium

Joined Jun 2, 2021
19
First, get all your design parameters together - what is the resistance of a snail?
Hard to measure. The first point of touch will be the tentacles which retracts fast on first touch and with a very small point of contact I don't know how much charge gets across.
 

Thread Starter

Paramecium

Joined Jun 2, 2021
19
Swap the inputs of the opamp and drive the monostable via a cap.
I'm afraid I need some explanation (with tea spoons!). You mean driving the monostable instead of the mosfet, but why swapping the inputs of the opamp?

(as a newbie it feels a be awkward asking questions and not grasping the answers : )
 

Ian0

Joined Aug 7, 2020
3,268
Isn’t this simpler than we think? How about a 555 (preferably a CMOS version) set for 5 seconds, then connect the snail between pin 2 and ground, and a pull-up resistor from pin 2 to positive equivalent to 2*R_snail?
It should track the battery voltage perfectly, because the threshold tracks the battery voltage.
Can’t you measure R_snail by setting the snail off across a piece of veroboard and measuring between two adjacent tracks with a multimeter?
 

ericgibbs

Joined Jan 29, 2010
13,652
hi Ian
If we used two dissimilar metals for the Snail sensor, and let the Snail 'slime' act as an electrolyte, perhaps we could get a few tens of millivolts generated.

E
 

Thread Starter

Paramecium

Joined Jun 2, 2021
19
Isn’t this simpler than we think? How about a 555 (preferably a CMOS version) set for 5 seconds, then connect the snail between pin 2 and ground, and a pull-up resistor from pin 2 to positive equivalent to 2*R_snail?
It should track the battery voltage perfectly, because the threshold tracks the battery voltage.
Can’t you measure R_snail by setting the snail off across a piece of veroboard and measuring between two adjacent tracks with a multimeter?
Thank you, I have tried. The sensor above works fast and at only a slight touch due to the op-amp great amplification. The one built wth the TS555 (and then the mosfet) does make the motor jerk a bit maybe, but does not trigger the 555 properly to get a 5 seconds run. It helps using as small pullup resistor as possible, but not enough.
 

Thread Starter

Paramecium

Joined Jun 2, 2021
19
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