Hello,

Here is the schematic with the schmit-trigger gates:

I have also attached the 40106 datasheet.

Bertus

 

Hello,

Here is the schematic with the schmit-trigger gates:

I have also attached the 40106 datasheet.

Bertus

That's simple and small. Here's another way.
I have included the LTspice .ASC file.

 

Hi Guys,

Thank you all every so much for your help here!

I have been away for work and got back to looking at this today so i will be getting the extra parts and then having a go to get it running.

Again, thanks!

 

Okay, so i designed and then built the second option simply due to having everything already.

For some reason its not working, i am putting it down to my lack of skill in going from schematic to build so i have attached pictures of the build so you can rip into it.

There is voltage running through the coil when the switch is thrown but only 0.1v so i suspect i have done something wrong.

Many Thanks,

 

It looks like the coil terminals are shorted. I didn't check the rest of it.

 

Yep, you are correct, i have drilled out one of the holes between the coil terminals now.

Unfortunately still doesnt work only getting 0.1v over the circuit.

Cheers, Scott

 

Yep, you are correct, i have drilled out one of the holes between the coil terminals now.

Unfortunately still doesnt work only getting 0.1v over the circuit.

Cheers, Scott

Is the 0.1V across the coil, or from the coil to ground? With the coil shorted, there is no current limiting for your MOSFET. It is probably burned out.

 

Okay, I thought it might be. I'll get a new one during the week.

Cheers!

 

What's the voltage needed to turn on the relay...is 9V enough??

 

That particular relay requires 70% of the rated voltage to pick up; or 8.4v.

If a brand-new alkaline PP3 9v battery were being used to power the circuit, it might work for up to a couple of minutes.

 

You are both completely correct, i have been checking it by connecting the battery directly to the coil terminals to check the battery is still outputting enough to throw the relay switch.

I am on my second battery already due to this.

Thanks again.

 

Hi Everyone,

So after spending some time looking at the circuit and then understanding LTSpice and the graphs you kindly created for me i think i might not have explained myself correctly.

From the look of the graphs (and please correct me if i am wrong) it seems to show that current will be running through the relay constantly then when the switch is activated at t=100 the circuit will keep the current running for what seems to be about 40 seconds then allow the relay current to reach zero until the switch is deactivated at t=200 at which point the current will return.

What i actually want is for no current to run through the relay as normal; then once the switch is activated current instantly runs through the relay and continues to run through it until the switch is deactivated. Once the switch has been deactivated i want current to continue to run through the relay for a further 30 seconds (ish) then drop below the required relay current.

I hope this makes a bit more sence for you now.

Many Thanks!!

 

What i actually want is for no current to run through the relay as normal; then once the switch is activated current instantly runs through the relay and continues to run through it until the switch is deactivated. Once the switch has been deactivated i want current to continue to run through the relay for a further 30 seconds (ish) then drop below the required relay current.

I hope this makes a bit more sence for you now.

Many Thanks!!

That's how both posted circuits work.

 

So, as expected i now appear dumb, i realised that the switch opened at t=0 not t=100. I also realised two of my resistors were placed wrong and now its been corrected the circuit works...

Except it doesnt throw the relay.

I changed to a 12v battery and the relay gets over 10v through it now but still doesnt throw.

If it remove it and connect the 9v battery to the terminals directly i can hear the relay switching but when its in the circuit it doesnt work even if i connect the battery directly to the terminals.

I can only guess that when its in the circuit that for some reason it isnt getting the full amount of amps???

BTW i changed the cap to 47uF (as i didnt have a 39uF) and the 1Meg resistor to 470k to give about 20 seconds of power.

Cheers guys!

 

The problem is usually due to a wiring error, or a bad part, often caused by a wiring error, or static discharge. Are you sure the MOSFETs are wired correctly?
MOSFETs are easily damaged by static discharge. Google "handling MOSFETs".

 

Hi,

Is there a way i can check a MOSFET with a multimeter?

All seems to be working though, if I measure the voltage across the terminals of the relay i get slightly over 10 volts when the switch is open and residual voltage for a few seconds of about 5.8v once it is closed. I think i need to work out the cap and resistor sizes to work properly as i think the cap doesnt have enough time to charge.

Cheers!

 

Hi,

Is there a way i can check a MOSFET with a multimeter?

All seems to be working though, if I measure the voltage across the terminals of the relay i get slightly over 10 volts when the switch is open and residual voltage for a few seconds of about 5.8v once it is closed. I think i need to work out the cap and resistor sizes to work properly as i think the cap doesnt have enough time to charge.

Cheers!

Measure the voltage between Source and Drain when the MOSFET is OFF, you should see close to the supply voltage.

Measure voltage between Source and Drain when the MOSFET is ON (after switch is closed to activate relay), you should see close to ZERO volts between the two.

With the power removed, measure the resistance of the relay coil, to make sure it didn't burn out.


I'm not sure if this is what the circuit still looks like, but it appears you have the coil between +V and the GATE of the MOSFET on the left.

 

Measure the voltage between Source and Drain when the MOSFET is OFF, you should see close to the supply voltage.

Measure voltage between Source and Drain when the MOSFET is ON (after switch is closed to activate relay), you should see close to ZERO volts between the two.

With the power removed, measure the resistance of the relay coil, to make sure it didn't burn out.


I'm not sure if this is what the circuit still looks like, but it appears you have the coil between +V and the GATE of the MOSFET on the left.

Thanks for that, i thought it would be best to write down my measurements so you can definately advise if they are as they should be.

When the switch is closed:
M1 between G+D is 9v and lowering
M1 between G+S is 9v and lowering
M1 between S+D is 0v

M2 between G+D is 9v and lowering
M2 between G+S is 0v
M2 between S+D is 9v and lowering

When the switch is open:
M1 between G+D is 11.4v
M1 between G+S is 0v
M1 between S+D is 11.4v

M2 between G+D is 11.4v
M2 between G+S is 11.4v
M2 between S+D is 0v

Relay coil resistance is measured at 695Ω (ish) and is rated at 760Ω so seems correct.

The Gate of M2 is between the relay and the diode, this seemed to be a logical way of doing it, is it wrong?

Cheers,

 

Thanks for that, i thought it would be best to write down my measurements so you can definately advise if they are as they should be.

When the switch is closed:
M1 between G+D is 9v and lowering
M1 between G+S is 9v and lowering
M1 between S+D is 0v

M2 between G+D is 9v and lowering
M2 between G+S is 0v
M2 between S+D is 9v and lowering

When the switch is open:
M1 between G+D is 11.4v
M1 between G+S is 0v
M1 between S+D is 11.4v

M2 between G+D is 11.4v
M2 between G+S is 11.4v
M2 between S+D is 0v

Relay coil resistance is measured at 695Ω (ish) and is rated at 760Ω so seems correct.

The Gate of M2 is between the relay and the diode, this seemed to be a logical way of doing it, is it wrong?

Cheers,

No, I think that's backwards. The Drain should connect to the coil/diode, the source to ground, and the gate to the voltage divider resistors and capacitors.

That would explain the problems you've had. Switch it around and it should be OK.

 

Hi There,

I have now switched it backwards so that the gate is to the resistor, the drain is to the relay and the source is still at ground.

Unfortunately its still not working, again over 10v measured across the relay but its not being thrown.

Many Thanks,

Scott