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Digital on/off timer circuit.

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simmisj

New Member
Hi.
I was trying to make a timer circuit (RC circuit) that turns a LED on after some time (just for test purposes I try to make it 10-20 sec).
I made this circuit but it does not work as a digital on/off circuit. It's more like a semi analog circuit. It charges for 15 seconds and then it starts to get brighter and brighter for 10 seconds.
http://www.northcountryradio.com/PDFs/column008.pdf
Can someone help me?
 

Martel

New Member
In the paper, there are many timer circuits (NPN + zener, op-amp and 555 timer).

Which one are you using ?
 

Reloadron

Well-Known Member
Most Helpful Member
Per Martel you don't mention which circuit you are using? If you are using the attached, what are the values or R and C and you do have the 9.1 volt zener in the circuit correct and the polarity of the zener is correct? The circuit running off 12 volts as it is drawn?

What you describe for symptoms sounds like the zener diode was omitted or is connected backwards.

Ron
 

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simmisj

New Member
Here is my circuit as is.

Hei.
I made a little drawing of my circuit and I will explain it now.

R1 is around 100k-500k (just enough to charge the capacitor in 10 secs or so).
R2 is just to protect the diode from to much current. 300-1000ohm.
C1 is 100uF electrocite. Purple on picture.
Q1 is a 2N3904 transistor.
Z1 is a 4.3V zener diode.

The drawing in the top of the picture is to show how the LED turns on. 6V being fully on and 0V being fully off. The RED spot on the circuit (made with paint) is where +6V goes and the GREEN spot is where the GND goes.
The circuit in action goes something like the RED line on my drawing. The function I want is the BLUE line.

I measured over the ZENER when I turned the circuit on and the voltage over it rises steadily.

Any idea what could be wrong? Any sugestions on another setup?

Thanks in advance.

Simmi

EDIT EDIT EDIT

The GREEN spot is wrongly put. That's not where the GND goes. The GND goes two points upper. To the emitter of the transistor NOT the collector.
 
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crutschow

Well-Known Member
Most Helpful Member
You need a circuit will more gain, such as those built with an op amp (used as a comparator) or a 555 timer. A single transistor does not have enough gain to give a digital snap action in your circuit.

The circuit you built just delays the initial part of the RC exponential ramp, but once the zener voltage is exceeded, then the later (slower) part of the exponential starts to turn on the transistor, slowly as you have observed.

Edit: Another way to provide a snap action is to use an SCR in place of the transistor. The power must be removed from such a circuit to reset the SCR after it is triggered. Use a sensitive-gate SCR to minimize the required gate trigger current.
 
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simmisj

New Member
So using an SCR instead of the transistor would do the trick? Couldn't I just use two transistors in a darlington setup?
 

crutschow

Well-Known Member
Most Helpful Member
So using an SCR instead of the transistor would do the trick? Couldn't I just use two transistors in a darlington setup?
That may work, depending upon how fast you want the LED to turn on.

You can also increase the transistor turn-on speed, while maintaining the same delay, by using a smaller resistor and a larger capacitor.
 

simmisj

New Member
Well, like I said before. I want the LED (in the end product it will be a LED and a buzzer) to turn on almost instantly. But over a few seconds is good enough.
Maybe if someone could explain what is happening in my circuit that makes it turn so "smoothly" on instead of just on/off I could have one of those very nice "aahhh that's the reason" moments. :)

EDIT

Maybe I said it wrong....
When the circuit is turned on I want it to wait for some minutes and THEN I want the LED to turn on almost instantly. :D
 
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crutschow

Well-Known Member
Most Helpful Member
Maybe if someone could explain what is happening in my circuit that makes it turn so "smoothly" on instead of just on/off I could have one of those very nice "aahhh that's the reason" moments. :)
The reason is quite simple. The transistor needs a certain base current to fully turn on and carry the LED current (required base current equals the LED current divided by the transistor beta gain). Any base current less than that and the transistor will have a lower collector current and the LED will only be partially on.

As the capacitor voltage initially exceeds the Zener voltage plus the transistor Vbe, a small base current will start to flow (equal to the difference between the capacitor and the supply voltage divided by the charging resistance). This current starts to turn on the transistor but is not enough to fully turn it on (since the Zener and transistor base both have a small series resistance and the Zener knee, --the point at which it starts to conduct --, is slightly rounded). As the cap voltage slightly increases, so will the base current which will eventually be large enough to fully turn on the transistor. This can take a noticeable amount of time, depending upon the transistor gain and the charging resistance value.

Is that clear enough?
 
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Martel

New Member
The Zener idea is not the best sine a Zener will start to conduct (a few microamperes) well below it's Zener voltage.

A much better idea would be to use a 555 timer. As soon as the RC will reach the threshold voltage, the 555 will output a HI instantly. Furthermore, if you use a LC555 (CMOS 555) and a good grade of capacitor (very small leakage) you'll be able to use a very long tme delay.

Another idea i used in the past and liked a lot is to use a 4060 CMOS IC.
 

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simmisj

New Member
Well yeah that was very well explained :) Thanks.

Dvinchy, I wanted to try to not use any IC circuits.
But I still tried with the 555 timer and the result was that it behaved in reverse. It started on and was on for the amount of time that I specifed. After that time it turned off.
I want it to be off for some amount of time and then turn on and be on until the circuit loses power.

I'm currently working on trying to get a voltage comparator working. I can't seem to find a good guide to the voltage comparators. I am using the LM311.
 

simmisj

New Member
Martel, didn't see your post before I posted the other one. Your post is the first on the second page. :)

Using the 4060 CMOS IC this would become to big of a circuit I think. There has to be a way to do it with 5-6 small components.
I will try the darlington way and the LM311.
The 555 is also a viable choice but I didn't seem to get the instructions that I read about setting it up in monostable mode. It works inversed of what I want it to :/
 

simmisj

New Member
Yey.
I managed to make the circuit I want with LM311 voltage comparator.
Project finished and running.
Thanks for the help everyone.


Simmi
 
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