# Noobie Alert! How does a TV On/Off Switch Work?

Discussion in 'General Electronics Chat' started by hellohellosharp, Nov 15, 2012.

1. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
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This might sound extremely noobish, but I don't understand the circuit behind a TV On/Off switch. When you push in and release, it turns on. When you push in and release again, it turns off.

This is different from a normal push button switch, where you push and it turns on, release and it turns off. According to my knowledge, a SIMPLE push button circuit would require you to hold the power button in while you watched TV.

Obviously, this isn't how it works, but I hope you get where I am coming from. I am looking for a simple circuit that simulates this (not necessarily a real circuit from a TV).

What parts are required to make a circuit where you push and release a button to change states instead of simply pushing?

2. ### Audioguru New Member

Dec 20, 2007
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A digital flip-flop IC can do it.
The on-off switch applies a pulse to it which makes it FLIP. When it flips then the TV turns on.
Another pulse from the remote causes the flip-flop to FLOP which turns off the TV.

The FLIP is a "set" and the FLOP is a "reset".

Apr 19, 2011
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4. ### Audioguru New Member

Dec 20, 2007
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No.
A flip-flop iC has a single "clock" input. The set-reset simple couple of gates you showed have a separate "set" input and a separate "reset" input.

5. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
27
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Ahh okay I see...

I think I am getting a bit closer? Please take a look at the link below...

The LED represents the TV and the push switch represents the On/Off switch...

The problem is the TV turns back off when the next clock pulses and the input is low. How do I get the TV to stay on?

6. ### Austin Clark Active Member

Dec 28, 2011
409
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You're getting closer. Behold:

With every clock cycle, the output wants to become the opposite of what it is at the moment. So it switches back and forth

EDIT: The clock would be replaced by the push-button by the way.

7. ### Ron H AAC Fanatic!

Apr 14, 2005
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What Audioguru is describing is a toggle flip-flop.

8. ### Austin Clark Active Member

Dec 28, 2011
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Commonly referred to as a T Flip-Flop.

There are many types of flip flops. The set-reset latch the op described earlier is the most basic building block.

http://en.wikipedia.org/wiki/Flip-flop_(electronics)

9. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
27
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Thanks so much! That is very simple and understandable

To test my knowledge, I want to make a push-button light switch. I am going to use the circuit and a relay to connect/disconnect the AC power from the light.

I want to use a circuit similar to the following:

Edit: Removed Link, see other one below.

The resistor represents the lightbulb.

The 150 OHM represents the lightbulb.

Would this circuit work? (I am actually going to make this as a project).

Also, would I need a special relay to handle the AC power?

Last edited: Nov 15, 2012
10. ### absf Senior Member

Dec 29, 2010
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Older non-LCD TV used push button locking switches like below...

They may come as locking or non-locking.

Allen

11. ### Austin Clark Active Member

Dec 28, 2011
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Excellent, you seem to be getting it.

Now, judging from your prior thread, you're going to take this a step further, and make a light that you can turn off and on remotely using a laser pointer and photo-resistor. That's actually an awesome project, and one that you've actually inspired me to try and build myself, so thank you for that.

Anyways, to start, you'll find that it's a little difficult to get the photo-resistor to turn off and on fully, to be used as the "clock" for your flip flop you've built. I'm sure there are many ways to accomplish this, but the way that immediately comes to my mind would be a comparator. A comparator is a circuit (That you can find pre-built inside chips) that outputs a full ON voltage whenever your input voltage is greater than a "threshold" voltage. Now, use the... Actually, here, let me just SHOW you what I mean...

This is what I came up with. Change the value of the "Photoresisitor" with the "resistance" bar. It's a potentiometer in the circuit, but I'm using it as a "Rheostat" (Variable resistor) which represents the photo-resistor.

12. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
27
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I guess you read my mind

I see what you are trying to do but for some reason adjusting the Potentiometer is not changing the status of the LED...

EDIT: Got it now, i thought you had it set to default is if the laser was on it, so I was moving it down instead of up.

What chips would this sort of circuit come in? Could I get this at radioshack?

13. ### Austin Clark Active Member

Dec 28, 2011
409
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The resistance must pass through the half-way mark and back to change the state of the LED.
Grab the bar with your mouse and move it back and forth a few times to see what I mean

14. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
27
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Yeah got it Seems to work perfectly for what I need.

I believe I have seen this sort of thing before with a transistor of some sort...

What chips would I get this in? Or should I just try and build it manually with the resistors and such.

15. ### Austin Clark Active Member

Dec 28, 2011
409
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You're probably thinking of a photo-transistor, which is another possible option, or a schmitt trigger, or something else completely.

The two chips you'd need are a comparator (or an op-amp, which can be used as a comparator), and a D flip-flop. And, of course, the resistors and such. I'd recommend a CMOS D flip-flop, like the 4013. http://www.datasheetcatalog.org/datasheets/228/108664_DS.pdf

As for a comparator/op-amp, pretty much any one will do just fine, beings that it's not being used at high frequency, or with particularly high voltages, or with high output current requirements, etc;

16. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
27
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Thanks again for your help I will be looking thru my electronic kits to see if I have these chips. Could you PM me your email? I would like to stay in touch with you as I go thru the project

17. ### shortbus AAC Fanatic!

Sep 30, 2009
4,096
1,691
Why not keep the thread going so others can benefit from your progress or problems? After all this is a form of electronic teaching site.

18. ### Austin Clark Active Member

Dec 28, 2011
409
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Of course I'll keep everyone updated.
I actually finished most of the project this morning. I couldn't find my laser pointer, so I used a flashlight instead for a proof-of-concept. Here's a short video of my results thus far:

19. ### hellohellosharp Thread Starter New Member

Apr 19, 2011
27
0
Hi Austin, I am having some trouble...

I am trying to make just the basic push-button switch mentioned earlier with the D Flip Flop. I am using the CD4013B type.

Here is the steps I did.

Connected Pin 14 (VDD) to 7.5 volts.
Connected Pin 2 (Q-) to pin 5 (D).
Connected Pin 7 (VSS) to ground.

Connected Pin 1 (Q) to a resistor, attached to an LED, to ground.

Now, i attached a probe wire into 7.5 volts (the LED is currently ON, attached to pin 1). I use the probe wire to touch pin 3 (clock). THis should turn off the LED right? But it doesn't...nothing happens.

I leaved 6 and 4 alone because I didn't know what they did...

Any help?

20. ### Pencil Active Member

Dec 8, 2009
271
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While you wait for Austin I thought I'd try to help.

See attached file for an example of what I think you want.
You want the configuration shown at the bottom.
Also note how unused inputs are connected, they should not
be left "floating".

Note, the 4000 series IC's have very little drive current
capabilities. Although they can drive an LED, but it is not
generally recommended. I would suggest you drive a transistor
or a MOSFET as a switch with the output of the 4013. This would give
you the ability to switch loads requiring much more current if needed.
If you need help just keep posting questions.

I have also attached a different datasheet for the 4013.

File size:
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• ###### 4013DATASHEET.pdf
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Last edited: Nov 17, 2012