555 timer HELP

[maverick28]

Hey Guys. SO i;m new to the forum and signed up to ask this.

I have a project where i'm modifying one of these

http://www.homedepot.com/p/BLACK-DECKER-Auto-Tape-Measure-ATM100/100081220#.Uj7tMT9afuY

Im building a timer for it where i press a button and the tape extrudes for a set amount of time and then stops.

*I'm Integrating a 555 timer monostable oneshot and have hit a cross road. I have set up this circuit



on my bread board. From what i believe it is set up correctly. So i am wondering if anyone can see a problem with this circuit not doing what i want it to do?

I want to be able to press a button the motor runs for a allocated time (dependent on the variable resistor) then turns off.

Thanks for any help and guidance.

cheers

 

[Harald Kapp]

this circuit not doing what i want it to do?

What do you want it to do? What does it do instead?
Ehat MOSFET are you using? Is this one suitable for 6V operation?

Further: The 555's out is high active. This means in the resting state it is low. This will turn on te PNP and thus the MOSFET (assuming it is a logic level MOSFET suitable for 6V operation). I'd expect the motor to be running all the time, pausing only while the monoflop is triggered.

 

[KrisBlueNZ]

As Harald said, the 555's output is normally low, and goes high during the pulse. So you need to remove the PNP transistor, and feed the 555's output to the MOSFET's gate. Usually a small series resistor e.g. 22 ohms is used, to reduce the peak current (the MOSFET's gate has significant capacitance). You can keep the 10k resistor from the gate to the 0V rail.

You can also delete R3 and C1, and connect the pushbutton from pin 2 to 0V. This means that you have to release the pushbutton before the pulse time expires, but I expect you would be doing that anyway.

As Harald also alluded to, you need to use a MOSFET that will saturate (turn fully ON) with only 6V of gate-to-source voltage. In fact, it will probably only see 4.5~5.0V because of losses in the 555. Not all MOSFETs will do this. Ones that do are called "logic level gate" MOSFETs, although this term is not exactly defined and even those MOSFETs will "saturate more" if given more gate voltage.

Here are a few suitable MOSFETs that are available from Digikey:

NTD4906N: http://www.digikey.com/product-detail/en/NTD4906N-35G/NTD4906N-35GOS-ND/2194521 USD 0.57; ON-resistance 0.008 ohms with 4.5V gate voltage; through-hole "IPAK" package.

PSMN022-30PL: http://www.digikey.com/product-detail/en/PSMN022-30PL,127/568-7512-5-ND/2606361 USD 0.73; ON-resistance 0.034 ohms with 4.5V gate voltage; standard through-hole TO-220 package.

FDP8880: http://www.digikey.com/product-detail/en/FDP8880/FDP8880FS-ND/976840 USD 0.92; ON-resistance 0.0145 ohms with 4.5V gate voltage; standard through-hole TO-220 package.

Lower ON-resistance values mean less voltage dropped in the device. For a small motor like this one, the difference is not important. But the TO-220 devices have the standard 0.1-inch pin spacing and are easier to use with breadboards and stripboard.

Edit: Oh, and welcome to the Electronics Point forums!

 

[maverick28]

Okay so what i want it to do is to be able to hit my button and the motor turns on for X amount of time then turns off.

 

[KrisBlueNZ]

Okay so what i want it to do is to be able to hit my button and the motor turns on for X amount of time then turns off.

I know. Your circuit will do that, with some changes. Read my previous reply. If you don't understand something, quote it and ask for clarification.

 

[maverick28]

thanks for your reply Kris, even though im struggling to understand most of it since im a rookie when it comes to this stuff. Was just wondering if you had some kind of schematic generator you could whip it up for me. As whatever you create i will try and build then come back with more issues or anything?

 

[KrisBlueNZ]

Open wide, here comes the train

 

[maverick28]

eeek

@kris can you read this forum for me and see what they are talking about. I pitched the same question to them and even posted what you wrote. Im really confused and have no idea what i need to do. His schematic looks a little different to yours and i think you have more idea what is going on !

http://www.electro-tech-online.com/threads/555-timer.138344/

 

[KrisBlueNZ]

Their advice is all good. Most of it agrees with my advice, just worded slightly differently. I think the problem is that you are not reading this advice carefully.

You've told them a lot of things that you should have told us earlier.

You didn't tell us that you were powering the circuit from four AAA cells. This affects things. You should use a low-power 555 such as a TS555 (STMicroelectronics) to minimise the load on the batteries while the circuit is idle.

The MBR735-60 diode you have is fine. On the schematic I posted before, I suggested a 1N4001, but the MBR735-60 is better, and if you have one, use it.

The IRF540N is not really suitable as the MOSFET. To make it conduct fully, the gate voltage needs to be more than 6V. Even using a modern 555 IC, this is not guaranteed, and as the batteries discharge, things will get worse. You need to use a MOSFET that has a "logic level gate" or "low VGS". I listed two alternatives on the updated schematic I posted before.

Their suggestion of adding decoupling capacitors across the supply rails is a very good one. You should add a 0.1 µF capacitor from pin 8 to pin 1 of the 555, and a larger electrolytic e.g. 470 µF across the battery where it connects to the circuit board.

I don't appreciate you playing off the other folks against me, and I'm sure they wouldn't either. You are wasting time for both of us.

You need to read the advice you are given. Read it carefully. If there's a word you don't understand, use Google and/or Wikipedia, or ask for clarification.

We (me, and the folks at the other site) are both trying to help you but you need to try to understand what we're saying. If you don't, you're going to have more problems when you come to build it up, and test it.