Seems dubious to me. On a 2.3V supply the diode will limit the gate voltage to 1.8V which is hardly enough to switch even a logic-level MOSFET.
Probably it's limiting the motor current and thus preventing the supply dipping as the motor starts.

It is going to be run on 3V (2 x 1,5V cells), but it is nice that it stills operates when the batteries goes a bit low. The 2,32 V value was just the lowest I could get it to work (with the motor) when testing it on my switching power supply.

 

hi Para,
In a colder climate, outdoors the batteries will be less efficient, use at least 3*1.5V cells = 4.5V

E

Added clip:
The performance of all batteries drops drastically at low temperatures; however, the elevated internal resistance will cause some warming effect by efficiency loss caused by voltage drop when applying a load current. At –20°C (–4°F) most batteries are at about 50 percent performance level.

 

hi Para,
Why are you using a 3V battery, which is requiring additional circuitry in order to make the project work,
a 4,5V or a 6V battery would be a better choice.
If its a 3v motor, use either a 3V battery for the motor.

E

I'm not sure if I follow you. I'm not using any additional circuitry (the diode and transistor is needed at 5V also) and the TS555 uses almost 20 % less at 3V compared to at 5V.

 

Hi, Para,
The point I am making is that if the circuit operation is marginal at room temperatures, it will most likely not work outside in the Norwegian climate.
E

 

How much current does the motor take?

 

hi Para,
In a colder climate, outdoors the batteries will be less efficient, use at least 3*1.5V cells = 4.5V

E

Added clip:
The performance of all batteries drops drastically at low temperatures; however, the elevated internal resistance will cause some warming effect by efficiency loss caused by voltage drop when applying a load current. At –20°C (–4°F) most batteries are at about 50 percent performance level.

It is actually an interesting question, what is best 2 or 3 cells?

The slugs will not be out when it is cold. The motor only loads a spring mechanism and does not need to be fast. The electronics use less at lower voltage. Instead of the extra cell I can have two larger cells. So why use 3 cells instead of just 2?

 

The slugs will not be out when it is cold.

Hi,
The slugs where I live usually come out at night or when its cooler.

When colder the battery voltage will be lower than your ambient test bench temperature

E

 

How much current does the motor take?

It varies with the load during one 6 sec cycle (1 loading of the springs) from about 80 to 120 mA.

 

hi P,
Do you have photo-shot of the project that you could post.?
It would help.
E

 

Hi,
The slugs where I live usually come out at night or when its cooler.

When colder the battery voltage will be lower than your ambient test bench temperature

E

It is a good point, and I am considering it – if needed. On the other hand; weak voltage is often used as a slug deterrent. The snail will creep over the first electrode and touch the second. If the is any deterrent in the voltage it will be less at 3V than at 4.5.

 

Wouldn't D1 be better used as a back-emf supressor across the motor? I don't see its purpose on the FET gate.

 

From my understanding, slugs avoid copper. No one has mentioned slug pellets, so I won't.

 

From my understanding, slugs avoid copper. No one has mentioned slug pellets, so I won't

Very clever. Metaldehyde,pyrethrin, are indiscriminate killers and reduce the biodiversity in a garden. The old double edged sword!
But then again the Spanish slugs gobble up the entire garden. Hydroponics lift them off the ground.

 

Wouldn't D1 be better used as a back-emf supressor across the motor? I don't see its purpose on the FET gate.

Well, I don't know why, but it is absolutely required make this thing work. Without it the motor only makes a small jerk each time I trigger. With it (and the resistor after it ) everything works perfect.

 

Without it the motor only makes a small jerk each time I trigger. With it (and the resistor after it ) everything works perfect.

Sounds like it's the slow decay of the FET gate charge via R5 that keeps the motor running long enough then. Increasing R6 and/or C2 would prolong the motor run time. Keep R2 though, as it ensures the FET goes fully off when the circuit powers down.

 

It is also possible to use that opamp as a comparator, which may draw less current. It should also be possible to use a CMOS gate as the sense amplifier, as the application requires either off or on and other conditions are of no interest. I have not worked out the circuit yet, but it would be rather simple, and use very little power. And an advantage of CMOS RC timing circuits is that the tip point stays very close to a fraction of the supply voltage, and so would be more stable as the battery ran down. And with no liner devices the battery drain would be just a few micro amps.

 

Adjust the position of the sensors relative to the cutter and it will not make so much mess. I wa going to suggest a mains power system to fry the slug, but those circuits are not allowed here. But frying might be less messy.

 

I would like to see a video of it, in action.
Good luck..