I'm using a 556 timer. Side A is a one-shot, side B is a an astable multi-vibe combination.
A 2.5-3 sec pulse cycle then output remains high until power is removed. Need this circuit to switch 12v via p channel mosfet witch requires neg input(right?) But the circuit output is pos. I don't know how to accomplish this. Logic gate? Help pleeeeze, thanks.

 

I'm using a 556 timer. Side A is a one-shot, side B is a an astable multi-vibe combination.
A 2.5-3 sec pulse cycle then output remains high until power is removed. Need this circuit to switch 12v via p channel mosfet witch requires neg input(right?) But the circuit output is pos. I don't know how to accomplish this. Logic gate? Help pleeeeze, thanks.

Not haveing a schematic of your circuit here is an idea. The NPN transistor such as a 2n3904 inverts the signal and when the collector is in saturation the P-channel mosfet is turned on.

 

Not haveing a schematic of your circuit here is an idea. The NPN transistor such as a 2n3904 inverts the signal and when the collector is in saturation the P-channel mosfet is turned on.

Your PMOS is upside down, and you definitely don't need negative feedback from drain to gate (the 100k) when making a switch.

JayK026, if I understand you correctly, you want the PMOS to be on when the 556 output is low. If this is correct, and your 556 is also powered by 12V, then connect the 556 output to the gate of the PMOS, the source to +12V, and the drain to your load, which I assume is grounded on the other end. If you are using a CMOS 556, then that's all you have to do. If you are using a bipolar 556, then you will need to add a resistor (1k should work) from the 556 output to +12V, because the output high level of the bipolar 556 will only go to about 10.5V, which is not high enough to guarantee that the PMOS is off.

If you want the PMOS to be off when the 556 is low, then you will need an inverter after the 556, as suggested by k7elp60.

 

Thanks for the responses, I should clarify my needs. So 12v is applied to the 556, at the same time 12v is ready to be switched through the pmos, the 556 output will ge high at this time. So the inverter would b the best option for this?

 

Thanks for the responses, I should clarify my needs. So 12v is applied to the 556, at the same time 12v is ready to be switched through the pmos, the 556 output will ge high at this time. So the inverter would b the best option for this?

This should work.

 

Your PMOS is upside down, and you definitely don't need negative feedback from drain to gate (the 100k) when making a switch.

JayK026, if I understand you correctly, you want the PMOS to be on when the 556 output is low. If this is correct, and your 556 is also powered by 12V, then connect the 556 output to the gate of the PMOS, the source to +12V, and the drain to your load, which I assume is grounded on the other end. If you are using a CMOS 556, then that's all you have to do. If you are using a bipolar 556, then you will need to add a resistor (1k should work) from the 556 output to +12V, because the output high level of the bipolar 556 will only go to about 10.5V, which is not high enough to guarantee that the PMOS is off.

If you want the PMOS to be off when the 556 is low, then you will need an inverter after the 556, as suggested by k7elp60.

As always Ron is correct. I was away from home on a trip with my laptop
and a while after I posted my reply I thought I forgot to invert the PMOS.
The 100K was susposed to be from the gate to source, to insure cutoff when the inverter wasn't on.
Thanks for the correction Ron.

 

Well, it works now! Switches 12v, I have this hooked to a brake line circuit. Works great when the vehicles ignition is on, but when running it dosn't function prop. Not sure why,
Some kinda interference? Any ideas?

 

Well, it works now! Switches 12v, I have this hooked to a brake line circuit. Works great when the vehicles ignition is on, but when running it dosn't function prop. Not sure why,
Some kinda interference? Any ideas?

When the engine is running, you are using what is probably the noisiest power supply you will ever work with.

 

Is that the issue I'm having...if so how can I isolate this circuit?

 

Can you tell us what the load is, and how much current it draws?

 

Is there a decoupling capacitor on the 556? You might also add a ferrite bead to the power input and possibly a second cap to form a pi-filter. Maybe you need to regulate down to 12 V or less -- your charging voltage may be much higher than you expect. Measure it.

 

Load led lights, 3.8 watts. Circuit is on motorcycle. Two separate switches, front and rear. While engine is running.... The rear brake switch triggers the circuit so it flashes 6 times then remains solid until released.
The front brake switch triggers the circuit but it only flashes once then remains solid. If I press the rear switch, then the front switch before the circuits cycle is through the tail light goes solid. The tail lights
Share the same input wire. Maybe a faulty wire in the front switch?

 

Hello,

I am closing this thread as it violates AAC policy and/or safety issues.

Quote:
6. Restricted topics. The following topics are regularly raised however are considered “off-topic” at all times and will results in Your thread being closed without question:

• Any kind of over-unity devices and systems
• Automotive modifications
• Devices designed to electrocute or shock another person
• LEDs to mains
• Phone jammers
• Rail guns and high-energy projectile devices
• Transformer-less power supplies

This comes from our Tos:
Terms of Service
There will be enough sites where automotive questions can be discussed :
Member selected automotive forums

Motorcycles are also considered as automotive modifications.

Bertus