# School Assignment: Delay On timer

Discussion in 'Homework Help' started by Woodbutcher, Dec 6, 2011.

1. ### Woodbutcher Thread Starter New Member

Nov 27, 2009
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I am in a beginner electronics course at jr high (9th grade) and am really liking it. We have learned about resistors, diodes, potentiometers, LED,s, transistors and scrs etc. Our final project is giving me trouble so I have been looking and looking and havent found what I'm looking for.

We get bonus marks if we can figure it out.My assignment at school has two parts to it:

1 to add a delay on circuit to a alarm circuit which uses an scr so that you can get out of the room without it going off. We are only allowed to use capacitors and resistors, etc., no 555 timers or IC's.

2 to add a delay circuit that lets you go in the room so I can shut the alarm off before it is triggered too.

The worst part is that I only have until christmas holidays to get it done.The class is really cool, I like my teacehr and I want to learn more. My knowledge is very limited. Any ideas?

Apr 5, 2008
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Hello,

I have moved your post to the homework help section.
Show us what you have done upto now and we will give you advice.

Bertus

3. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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How much time do you need to get out of the room after arming?

How much time do you need to get into the room and disarm?

What is the trigger voltage on the gate of the SCR?

Do you know what an RC time constant is?

If you can answer all of the above questions, you have your solution.

Dec 26, 2010
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You might also like to get a value for the minimum gate trigger current required by the SCR. If that current is not particularly small, you may find that a transistor would be helpful in your solution, depending whether a transistor is allowed.

5. ### Woodbutcher Thread Starter New Member

Nov 27, 2009
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I have two pictures to show.One is the original schematic from class. The other is my ideas using a scr, a pot and a capacitor. I havnt figured out values for the parts yet. The scr is a C106B1. the pot is 100k and the diodes are 2n4007. I think we have 3904 transistors at scchol too.

Thankyou very much for the help.

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• ###### alarm with delays.pdf
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Dec 26, 2010
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As it stands, the circuit you show cannot give a delay. A capacitor in series with the gate would let current pass immediately, but this would reduce as the capacitor charged. If I understand your requirement correctly, more or less the opposite is required. Can you think how this could be done?

Also, have you been asked to use an additional SCR, or are you supposed to alter the way the existing one works?

Dec 26, 2010
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You will of course understand that we can't directly answer this question for you. Does it help you to think that the voltage across a capacitor increases with the charge passing into it? Therefore, if a current is passing into a capacitor, the voltage across it will progressively change.

The C106B1 requires typically 15μA, worst-case 500μA to trigger it, at no more than 1V, and no less than 0.4V at room temperature. If the SCR is very hot, the voltage may be as low as 0.2V See datasheet:

http://pdf1.alldatasheet.net/datasheet-pdf/view/11640/ONSEMI/C106BG.html

Given the wide range of the trigger requirement, it might be better not to use the trigger sensitivity to determine the timing.
You seem to be at an early stage though, so perhaps only a simple answer is expected.

8. ### Woodbutcher Thread Starter New Member

Nov 27, 2009
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I don't have to use an scr, i just figured since I could see how it worked for triggering the alarm it might work for the on-delay b/c I want current to flow into the rest of the circuit after time has gone by.

Should I have the capacitor and resistor in parallel?

or maybe the capacitor should go to ground?

or maybe the negative side should go to ground and the gate thru a diode so it will charge the capacitor then send current to the gate. Will the diode act as a bit of a resistor untill the capacitor has done its thing?

I know you cant just give me the answer, so I am trying to understand this. Even if I dont get the assignnemt done in time I still want to figure this out.

9. ### Woodbutcher Thread Starter New Member

Nov 27, 2009
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ajuster I read your answer and I can see why the scr would let current thru right away so I wokred on it some more.

A friend said that I needed ao on off switch that would let the capacitor discharge when i turn the circuit off so I added that (i think its right).

I tried a couple of ideas but I think idea 3 is the best. The capacitor needs to be grounded I think and then let the current go to the gate of the scr. In the alarm circuit a diode was between the NC loop and the gate (Im thinking to stop current going to the gate untill the NC switch is opened giving no other way for the current to go expect thru the gate - is that right?)

I hope im on the right track.

• ###### on delay 1.pdf
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Dec 26, 2010
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A discharge path for the timing capacitor is a good suggestion. Be sure that it won't prevent charging when this is required though.

Timers often work by charging a capacitor until the voltage across it reaches a threshold value. The capacitor must be charged at a restricted rate, so that the voltage across it builds up slowly. The voltage across it needs to reach something that will sense when it has got big enough.

The idea of going via a diode into an SCR gate is along the right sort of lines, but note that the SCR might need 1mA of gate current if we take the highest specified value. Typically a lower current is needed, but a safe design would require the current charging the capacitor to be at least 1mA, so that you would need a capacitor that would take about 12 seconds to charge through something like 1.2V at 1mA.

If the capacitor is charging at a minimum of 1mA for about 12s, we can estimate its minimum value. For a capacitor, charge Q in coulombs (aka amp-seconds) = voltage V in volts * capacitance C in farads. Q = CV, hence C = V/Q

For a constant current of I amps flowing for T seconds, Q = IT, so C = V/IT = 1.2V/(12s*0.001A) = ??? ...A bit big, eh?
(The current would not be quite constant if charging was via a resistor, but this gives a rough estimate.)

The circuit could be built like this, but is there any way you could imagine a smaller capacitor being used? Think about it, I must go now.

Dec 26, 2010
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EEK, your arrangement (2) will certainly give a time delay, but the SCR is shown across the supply. If it's left like that, it will short out the battery.

12. ### thatoneguy AAC Fanatic!

Feb 19, 2009
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#3 is close to what I was thinking. The alarm would just be a wire loop that is broken by various switches, when that happens, the SCR would turn off, so if you had a buzzer from the cathode of the SCR to V+, it would buzz when the arm switch is on, but the alarm wire/loop was broken.

To reverse that, using the same SCR, would be a little different, though, since opening the door would open the loop (magnetic reed switch), it needs to wait 10 seconds before going off. How would you add that time delay to circuit #3 with the buzzer connected the way I described? (SCRs need a minimum of about 5mA current to "hold" them on once activated)

13. ### Woodbutcher Thread Starter New Member

Nov 27, 2009
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I was sick all week so I couldnt work on the assignment. It is due tomorrow so I dont think I will get it done. Thankyou for all your help.

I would still be intrested in seeing how this circuit would work. I can work on it over christmas holidays.