Pin 7 of 555 timer doesn't discharge

[JimHebert]

Hi Y'all:

I am trying to use the 555 in a monostable mode. I trigger the timer on and output stays high for duration of 1.1RC =T. All is fine. Pin 3 (output) goes low when Pins 6 reaches 4.6 V (2/3 Vcc =6.6v) and 7 (discharge) for some reason does not discharge the timing Capacitor at that instant and pins 6 and 7, which are connected together, remain at or a little lower than the 4.6V level. I then short the trigger to ground again and the output goes high again but since the timing capacitor is near threshold voltage, the output pulse width is very short. The only way I can get it to time out properly is to disconnect the circuit from Vcc and let the RC circuit bleed down on its own for a few minutes and then I can reconnect Vcc and get it to work and give me one and only one proper output pulse width. I jiggle the chip in the socket, in case of a bad connection but it doesn't help.

 

[(*steve*)]

can you post your circuit diagram.

 

[JimHebert]

I have uploaded the circuit file but doesn't appear on this thread. I hope you received it.

 

[JimHebert]

I now see the file has posted. Thanks.

 

[Sadlercomfort]

I see you've written 0.1uF where there should be a cap.. Pin 5 should not be connected to ground.

Also, check your relay doesn't draw anymore than 200mA otherwise this should be switched using a transistor.

I'm not entirely sure how the trigger circuit functions, but you could use a 1MΩ resistor from the cap to ground, to allow the capacitor to discharge again.

 

[JimHebert]

Pin 5 is connected to a .1 ufd and then it connects to ground. Pin 5 does not connect directly to ground. Slight error in my drawing.

The coil has 100 ohms. Should not draw more than 6.6V/100 = .066A....very little.

I don't understand why I should need a 1Meg bleed resistor. Shouldn't pin 7 discharge (go low - ground) and stay low as long as the output pin 3 goes low and stays low until the next trigger pulse (pin 2) is applied? If pin 7 doesn't remain at ground simultaneously with the low output, the capacitor will charge up without having a trigger pulse. Not good.

 

[(*steve*)]

what is the value of the cap marked as 22kufd?

if it is greater than some small value (it is mentioned in the datasheet) then you risk destroying the discharge transistor in the 555.

I suspect this may be the cause of your problem.

 

[Bluejets]

The other thing to watch is that your trigger time on pin 2 is less than your overall time.

 

[JimHebert]

what is the value of the cap marked as 22kufd?

if it is greater than some small value (it is mentioned in the datasheet) then you risk destroying the discharge transistor in the 555.

I suspect this may be the cause of your problem.

The capacitor (5 of them in parallel) is equivalent to about 22000 ufd. You are probably right. I didn't know there was this limitation, but it stands to reason. My goal is to have a long (4 or 5 minutes) time constant. I had previously tried using a smaller 470 ufd capacitor and a very large 200Meg ohm resistor to give me a 4 or 5 minute T = 1.1RC, however in this case the resistor (pot) was too large to sink enough current through the emitter-base of the internal threshold transistor (pin 6) to turn it on, which normally results in the output (pin 3) go low. What a dilemma. Any suggestions on getting a long time constant using this 555 timer?

 

[JimHebert]

The other thing to watch is that your trigger time on pin 2 is less than your overall time.

Yes , the trigger pulse is less than the output pulse.

 

[(*steve*)]

Aaaah, long periods and the 555. I'm sure we have a resource about that.

The 555 is really not suited to it. There are other, better methods.

 

[JimHebert]

Aaaah, long periods and the 555. I'm sure we have a resource about that.

The 555 is really not suited to it. There are other, better methods.

Do you have any particular circuit that you would recommend? I am an old analog technician and kind of like RC circuits, although there are digital counter circuits that can be used also for timers, but I am kind of rusty on the digital and Boolean algebra. It's been quite a while since I fooled around with that. But I am open for anything. I just clicked on your "resource" link. Thanks.

 

[(*steve*)]

Do you have any particular circuit that you would recommend?

Possibly

What are you trying to do? The solution may need to be tailored to it.

Practically speaking, it may be simpler to use a microcontroller. A PicAXE can be obtained in a package the same size as a 555 and probably do what you want with no external components! (other than a couple of resistors to allow programming)

 

[JimHebert]

Possibly

What are you trying to do? The solution may need to be tailored to it.

Practically speaking, it may be simpler to use a microcontroller. A PicAXE can be obtained in a package the same size as a 555 and probably do what you want with no external components! (other than a couple of resistors to allow programming)

Last year I had a 110VAC siren alarm wired up to the contacts of a DC activated relay. The relay was activated (contacts closed) by a DC cell phone adapter charger which was plugged into a Home Depot motion sensor. It worked fine until one day the motion sensor didn’t turn off and the siren stayed on for who knows how long before burning up the winding inside. So now I wanted to make a timer that would turn off the alarm even though the motion sensor stayed on.

I never heard of a PicAXe but willing to consider it. Tell me more.

 

[CDRIVE]

If you go with Picaxe I'll design the circuit and write the code for you. Picaxe will open a whole new world to you!

Chris

 

[BobK]

My wife does a lot of work with a picaxe, but I only get involved when a Kubota is needed:

 

[JimHebert]

If you go with Picaxe I'll design the circuit and write the code for you. Picaxe will open a whole new world to you!

Chris

I appreciate the offer ,Chris. I am sure you charge for your services. It may also be better that I learn to program it myself in case I want to change the program in the future to accomplish another task. You may not be around.

 

[CDRIVE]

I appreciate the offer ,Chris. I am sure you charge for your services. It may also be better that I learn to program it myself in case I want to change the program in the future to accomplish another task. You may not be around.

No, I don't charge for anything. Here's the link to the Picaxe home page.
http://www.picaxe.com/
Here's the link to the Picaxe 08M2 8pin DIP.
http://www.picaxe.com/Hardware/PICAXE-Chips/PICAXE-08M2-microcontroller
Here's the link to download the FREE software..
http://www.picaxe.com/Software
If you own a USB/Serial adapter dongle you don't have to buy the Picaxe programming cable. On the other hand if you own a PC with a real Serial Port you don't need a USB/Serial converter at all. That said the Picaxe cables are nice (molded) compact designs with a 1/8" stereo plug mounted on the Picaxe end of the cable.
http://www.picaxe.com/Hardware/Cables/

If you have any questions just post it to this thread.

BTW, where do you think I'm going? Do you know something I don't??

Chris

 

[(*steve*)]

Thanks for the information Jim.

Is there a source of 5V in the equipment you have? Or maybe 12V? -- the PicAXE requires 5V, and if this is available it removes the need for a regulator.

Also, what exactly is the nature of the input as it may require some conditioning.

finally, we need to consider the required output. Whilst this will mirror the input, is it power to the alarm or a signal? What voltages/currents are required?

 

[CDRIVE]

Steve, many of the Picaxe family will now operate down to 3V. Even the older chips ran fine on 3 x 1.5V cells.

Chris