555 timer

[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

**broken link removed**

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

 

[ericgibbs]

Hi and welcome,
Why are you inverting the 555 output to the NMOS.?
E

 

[Maverick28]

I was told to either invert and use NMOS or use a PMOS



Thanks for quick response! i have a week to get this thing working, im only new to breadboards.

I have lost confidence with these schematics as i have built both and nothing has come from either of them. Have even replaced resistors/caps/pots/555 and still nothing

 

[ericgibbs]

hi M,
With a suitable NMOS the high going timed pulse on pin#3 of the 555 will drive the NMOS into conduction.

Ideally the NMOS should be a low Vds switch ON, say switching on at 1or 2V and being harder on at ~+6V
Which NMOS type do you have.?
E

 

[Maverick28]

E!

thanks for all this.

http://www.irf.com/product-info/datasheets/data/irf540n.pdf

M.

 

[ericgibbs]

hi,
Look at page2 of the datasheet, Vgs threshold can be as high as 4V, which means at ~+6Vgs the NMOS will not be hard on.
What current does the motor require to Start and Run.?
E

 

[Maverick28]

Starting draw is at 1.297 A
and after 5 seconds due to more load on the motor it gets to 1.7223 A

M

*edit OR the batteries are running dry.

 

[ericgibbs]

Starting draw is at 1.297 A
and after 5 seconds due to more load on the motor it gets to 1.7223 A

M

hi,
I would say that for that level of load current the IRF540 should be OK.
If you wish I could run some LTS simulation to check the circuit out.
E

 

[Maverick28]

If you could that would be awesome. I am really stumped and i have rebuilt this thing that many times that i am starting to believe the circuit is missing something.

 

[ericgibbs]

If you could that would be awesome. I am really stumped and i have rebuilt this thing that many times that i am starting to believe the circuit is missing something.

OK, give me a few minutes.
The other point you should consider is that the circuit will require decoupling capacitors on the +6V rail, say a 470uf or higher and a couple of 100nF caps near the 555.

 

[MikeMl]

Eric nailed it.

As the AAs wear out, your circuit will have to work on ~4.5V. You will have to use a CMOS 555 timer chip (which works down to 3V), and a modern "logic-level" NFET.

 

[Maverick28]

Thanks Guys, good to get some guidance.

I'm running a NE555--- https://www.ti.com/lit/ds/symlink/ne555.pdf


so you recommend a get one of these --- https://www.ti.com/lit/ds/symlink/lmc555.pdf

i dont mean to be rude, but its 1220am here at the moment, i will be on first thing in the morning to reply to your posts. Thanks for all the help so far!.

**its running on 4xAAA

 

[ericgibbs]

hi,
This is a sim using a CMOS and 555 TTL timer and as Mike points out, using batteries I would consider at least a CMOS version and a lower Vgs MOSFET
E

NOTE: the lower plotted traces, the true voltages are not scaled correctly, they just show the relative timings...OK?
The IRF530 type is the closest I have for simulation..

 

[alec_t]

1.7A from 4 x AAA? What type? Alkaline, NiCd, ..... ? Battery life will be short.

 

[ericgibbs]

1.7A from 4 x AAA? What type? Alkaline, NiCd, ..... ? Battery life will be short.

The OP is using 4*AAA for 6V battery supply, a NiCd is a nominal of 1.2V/cell... so my best guess is Alkaline.

 

[alec_t]

Oops. Missed the 6V bit

 

[audioguru]

The simulation (and graphs on the datasheet) of a Mosfet are only for a "typical" device. A passing but less sensitive one probably will not work.
When the 6V battery runs down to only 4V then a logic-level Mosfet (IRL540) is needed.

 

[Maverick28]

Alright @E, thankyou for that, i will have a go at building the circuit you created, it seems that you have gotten rid of the inverter, PNP and diode.?

I'm a rookie so will ask a few silly questions so bare with me mate!.

1. Is 'vm' the motor?
2. Is r6 the POT?
3. what is V2?

It runs on alkaline and does not take long at all before they are dead flat.

Thanks for all your input!

 

[ericgibbs]

hi M,
The diode across the motor is required to protect the NMOS against inductive 'spikes' due to switching the inductive load of the motor, R4 represents the motor load in the sim.
The Inverter would have been required had you used a PMOS FET.

vm is a plot of the voltage on the Drain of the FET, ie: bottom end of the motor.
R6 represents on the sim your 100K pot set to half way ie: 50%
V2 represents your push button in the sim.
OK.?

AAA will soon be 'spent' when a current of ~1.7Amps is drawn from the battery.

E

 

[Maverick28]

one of my lecturers sent me this.so what do you think?


"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.

Also 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: https://www.digikey.com/product-detai...GOS-ND/2194521 USD 0.57; ON-resistance 0.008 ohms with 4.5V gate voltage; through-hole "IPAK" package.

PSMN022-30PL: **broken link removed** USD 0.73; ON-resistance 0.034 ohms with 4.5V gate voltage; standard through-hole TO-220 package.

FDP8880: https://www.digikey.com/product-detai...80FS-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."