# Touch operated switch

Discussion in 'Homework Help' started by arun the curious, Jul 27, 2013.

1. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
How to develop a touch operated switch with these things
1. 741 ic
2. 7473 flip lop ic
3. transistor-sl100
4. relay
5. resistors.

i hope someone knows the circuit...

2. ### LDC3 Active Member

Apr 27, 2013
920
161
I'm sorry but the nanny left earlier this year.
Since this is your homework, don't you think it would be wise for you to attempt a circuit so we can tell you what might need some tweaking?

3. ### ScottWang Moderator

Aug 23, 2012
6,589
1,021
741 → 7473 → resistor → st100 → relay
To drawing the function of 741.
To figure out what's the function that 7473 should be at here.
The resistor, transistor and relay, they are just a normal driver circuit, You will be very easy to find it from google.

Post your circuit when you got the idea, then everybody will give you the properly suggestions,

arun the curious likes this.
4. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
k...i attempted to do it.
741 : - connected to grnd
+ connected to a wire where we can touch.
6v supply given
output of 741 connected to 7473 ic's clock and 7473 ic set in toggle mode with j and k pin shorted to 5v supply.
From the output of 7473, put a resistor- 3.9 k ohms and connect to the base of sl100 transistor.
Connected the relay to the collector of trnstr with emmiter grounded. Also a diode was connected across the relay.

i googled for the ckt but cudnt find sny such ckt...

5. ### LDC3 Active Member

Apr 27, 2013
920
161
It sounds like it should work.

Since you can't find a circuit, maybe you can draw one for us to check it over.

6. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
sry...how to draw one?

7. ### blueroomelectronics AAC Fanatic!

Jul 22, 2007
1,758
98
You could try circuitlab.com
It has a 741 & JK flipflop plus a simulator.

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8. ### LDC3 Active Member

Apr 27, 2013
920
161
You can draw it on paper, and then either take a picture or scan it. You could try using Paint, or a similar program. Or you can use an online schematic editor (such as this) to make your circuit. If you use the online editor, you will need to create an account (for free) to save your schematic.

arun the curious likes this.

Aug 6, 2012
60
3
here is it

File size:
288.8 KB
Views:
35
10. ### LDC3 Active Member

Apr 27, 2013
920
161
Currently, when the sensor is touched, nothing happens. Since you are using a 741, a change in voltage level is needed. How can you accomplish that?
Instead of fixing the -ve input to one voltage level, it would be better to have voltage divider.

The rest is good.

11. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
Sorry. Didnt really understand what u meant.
Is it that i should change the negative input from being grounded to some other voltage level?

12. ### LDC3 Active Member

Apr 27, 2013
920
161
Suppose when you touch the switch, the +ve input changes from +6V to +4V. The output of the op-amp changes from +6V to +4V. This change is not enough to clock the flip-flop.

You want to be able to control the change in voltage from the touch switch and be able to adjust when the op-amp provides enough of a change to clock the flip-flop.

It would probably work the way you have the -ve input, but it would be better if you could set the -ve input to a voltage level between +6V (+5V is better for the 7473) and -6V (0V is better for the 7473).

13. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
Sorry ldc3...but im not understanding what u r intending to say...im relatively newbie so please share the correct ckt or say what i shud do exactly.
Doesnt u mean the inverting terminal by saying -ve input?
Voltage generated when u touch is very small,so making it to compare with the a higher second input will not produce any output...so that is y grnded the inverting(-ve) terminal.

14. ### LDC3 Active Member

Apr 27, 2013
920
161
Yes
I don't think you can rely on a voltage being sensed when you touch it. It is better to connect the line through a 1MΩ resister to +6V. Your finger then connects this line to ground. If you look at the touch circuit "Standard Touch Switch by Gate of IC 4011" on this page, you will see that the touch changes the voltage from +6V to near 0V. This is why you might want the -ve input to be adjustable since the touch input may not go to 0V.

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15. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
Thanks for the replies...but can u please share the circuit...?

16. ### ScottWang Moderator

Aug 23, 2012
6,589
1,021
What's the purpose of this project(just for fun, for home control, school home work)?

17. ### arun the curious Thread Starter Member

Aug 6, 2012
60
3
not just for fun...it was my sort of work given from my school long before but cudnt show the output then...now i want to make sure exactly how to make it...to find what exactly i went wrong...To learn it.

18. ### LDC3 Active Member

Apr 27, 2013
920
161
You should have told me this earlier, I could have made it easier for you.
Here is the input for the touch that I was trying to get from you. Oooops, I forgot that the op-amp has a -6V rail. Change the voltage for the op-amp and resistor for the -ve input and have the touch sensor connected to ground.

• ###### Touch-Input.png
File size:
11.4 KB
Views:
26
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19. ### LDC3 Active Member

Apr 27, 2013
920
161
Updated schematic.

• ###### Touch-Input.png
File size:
11.7 KB
Views:
13
arun the curious likes this.

Aug 6, 2012
60
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