74C14 Touch Switch - Togle Switch ON/Off

[prprog]

I found this circuit on the web, I give a try by adding a transistor switch at the ouput with an LED to indicate state (On/OFF) and it did not work the first time. I change the Capacitors to a 101 ceramic disc cap and a 10uf polarized cap. The power comes from a 9 volt baterry. The circuit works better, still the ON/OFF toggle fuction does not work consistently and when it is off it stay off only for a few second, then it will turn on again. If it on the ON state it will remain ON, That is OK. If I kept the fingers on the touch plate the LED blinks , like the circuit is oscillating. That behaviour is great and I want to be that way. How can I make the circuit more stable in the ON/OFF behaviour, again it is not precise to get an ON or OFF state.
Thanks

 

[Reloadron]

I see the Vcc connection which should be pin 14 of the 74C14 but where is Ground which is pin 7? Additionally you have the 74C14 which is a Hex (six) Schmitt Trigger. Where are all the unused inputs tied? Finally it looks like the circuit came from this link with the operation explained. Have you tried building the improved version?

Ron

 

[prprog]

I see the Vcc connection which should be pin 14 of the 74C14 but where is Ground which is pin 7? Additionally you have the 74C14 which is a Hex (six) Schmitt Trigger. Where are all the unused inputs tied? Finally it looks like the circuit came from this link with the operation explained. Have you tried building the improved version?

Ron

Thanks Ron - Yes I try the improved version but the behavior is the same. Can you explain more on the unused inputs ? I am using Pins 2 y 3 but the other are not connected to anything. On my breadboard pin 7 is the negative and pin 14 is the positive on the 9 volt battery. If they remain unused to what should I connect those pins? ground? and would that mean all 10 remaining pins?

Thanks a lot

 

[Reloadron]

Ground the unused inputs. So for example if you are using pin 1 in and pin 2 out (pins 1&2 used) you want to ground pins 3, 5, 9, 11 and 13 which are the remaining 5 unused inputs. See if that makes a difference in the operation.

Ron

 

[prprog]

This is the same circuit with the transistor , resistors and the LED. I put the inputs to ground but the behavior on the OFF state is the same; it will turn OFF but only for a few seconds. Also touching the plates does not always turn off or on the LED. It is not consistent. (I also retry the Improved version on the web site but it is the same as the simple circuit)
Thanks a lot for your help.

 

[ccurtis]

At a minimum, you should put a resistor in series with the base of the transistor before connecting it to the rest of the circuit, say 10 Kohms. Otherwise, the transistor is bucking the soft signal on the right side of the 1M resistor, preventing the circuit from working properly. Better to use the improved version of the circuit to drive the transistor to better isolate the touch plate from the transistor's base drive, and you should still put the resistor in series with the base of the transistor. Better even yet, use a third inverter of the IC to drive the transistor, alone, again putting a resistor in series with the base of the transistor.

 

[ljcox]

Here is my suggestion.

You also need the bypass capacitor C3.

I chose the resistor values to give a LED current of about 7 mA.

I assumed that it is a red LED.

 

[prprog]

ljcox

[first of all , I press the negative sign on the "Did you find this post helpful? by mistake my intension was to press +++ but it is so small that I made a mistake. Again sorry. It was indeed helpful]

I built the circuit as proposed, still it is not consistent in the ON/OFF behaviour. The LED will not stay ON for a long period. Is this the normal way of working of such circuits? My idea is that since it is a toggle switch it will remain ON or OFF until the touch plates are activate then it will change state. Also I should note that with the Capacitor values suggested it does not do nothing . I used a 101 cap and a 10uf polarized cap to get the results already indicated.

[I am probably wrong, but since the behaviour is link to the capacitors choosen can the Breadboard I am using make the circuit fail becuase it is "like"a capacitor ?- actually I read somewhere something about this but electronics is not my expertise area so I cannot said this is true]

More suggestions please.

Thanks a lot, I am learning a lot.

 

[alec_t]

The circuits in posts 5 and 7 have no pull-up or pull-down resistor on the input of the (first) logic gate. As a result the capacitor at the input will gradually charge or discharge so the LED will not stay in a latched state.

 

[ljcox]

ljcox

[first of all , I press the negative sign on the "Did you find this post helpful? by mistake my intension was to press +++ but it is so small that I made a mistake. Again sorry. It was indeed helpful] No problem

I built the circuit as proposed, still it is not consistent in the ON/OFF behaviour. The LED will not stay ON for a long period. Is this the normal way of working of such circuits? My idea is that since it is a toggle switch it will remain ON or OFF until the touch plates are activate then it will change state. Also I should note that with the Capacitor values suggested it does not do nothing . I used a 101 cap and a 10uf polarized cap to get the results already indicated.

[I am probably wrong, but since the behaviour is link to the capacitors choosen can the Breadboard I am using make the circuit fail becuase it is "like"a capacitor ?- actually I read somewhere something about this but electronics is not my expertise area so I cannot said this is true]

More suggestions please.

Thanks a lot, I am learning a lot. You're welcome

The point made by Alec_t occurred to me also after I posted the circuit.

It is a very crude circuit. I just tried to measure the resistance of my thumb.

The results were inconsistent.

On the 200M scale, I read about 11M.

But there was no reading on the 20M scale.

So I tried another meter which is auto ranging & it gave about 12M.

I suggest that you measure your own skin resistance by connect a meter across the touch plate & tell me what you read. Do it without the power connected to the circuit or alternatively, disconnect one side of the TP.

 

[prprog]

The circuits in posts 5 and 7 have no pull-up or pull-down resistor on the input of the (first) logic gate. As a result the capacitor at the input will gradually charge or discharge so the LED will not stay in a latched state.

alec_t -> Can you explain further how do I connect a pull-up or pull-down resistor on the input of the first logic gate. What would be a correct value of such resistor?

Thanks

 

[alec_t]

Use the circuit which has two gates. The second gate supplies positive feedback via a resistor to the first gate and so pulls the first gate input up or down as necessary.

 

[prprog]

Use the circuit which has two gates. The second gate supplies positive feedback via a resistor to the first gate and so pulls the first gate input up or down as necessary.

alec_t - Can you post your recomendation as a drawing (used one of the circuit drawing in this threat)? To be honest I do not undertand your suggestion (...becuase of my lack of knowledge). Thanks.

 

[alec_t]

Ron gave you the link in post #2. Here it is
http://talkingelectronics.com/projects/TouchSwitch/images/Toggle_touch_sw3.gif

 

[ljcox]

That is a better circuit, but its operation still depends upon your skin resistance.

As I said in my previous post, I measured mine at about 12M.

This would not be sufficient to pull down or pull up the input to the first Schmitt Trigger to the point where it can toggle.

 

[prprog]

That is a better circuit, but its operation still depends upon your skin resistance.

As I said in my previous post, I measured mine at about 12M.

This would not be sufficient to pull down or pull up the input to the first Schmitt Trigger to the point where it can toggle.

First thanks to ljcox , alec_t, reloadron and ccurtis for you comments and advice. I considered the skin resistance variable and decided to went back to the circuit in post #1 (original) change the resistor from 1M to 10M and test some caps combinations. I ended using 104 capacitors. The circuit work as expected. Now it toggle ON/OFF consistently when the plates are touch. (it might have a very, very small delay on the OFF event but it is acceptable)

Again, thanks.

 

[ljcox]

First thanks to ljcox , alec_t, reloadron and ccurtis for you comments and advice. You're welcome.
I considered the skin resistance variable and decided to went back to the circuit in post #1 (original) change the resistor from 1M to 10M and test some caps combinations. I ended using 104 capacitors. The circuit work as expected. Now it toggle ON/OFF consistently when the plates are touch. (it might have a very, very small delay on the OFF event but it is acceptable)

Again, thanks.

You don't have a resistor to define the input voltage.

As it is, this voltage will be determined by the leakage current of the IC & the leakage resistance of the 100 nF cap.

Both will be small, but if you leave the circuit powered for several days, you may find it has changed state.

That is why the circuit with a 4M7 feedback resistor is better.

However, I don't know if you want it to toggle or not.

In other words, do you want to be able to switch the LED on & then press the TP later to switch it off?

Or do you want the LED to be on only while you're pressing the TP?

 

[prprog]

You don't have a resistor to define the input voltage.

As it is, this voltage will be determined by the leakage current of the IC & the leakage resistance of the 100 nF cap.

Both will be small, but if you leave the circuit powered for several days, you may find it has changed state.

That is why the circuit with a 4M7 feedback resistor is better.

However, I don't know if you want it to toggle or not.

In other words, do you want to be able to switch the LED on & then press the TP later to switch it off?

Or do you want the LED to be on only while you're pressing the TP?

....to be honest...I did not undertand your explanation (thanks anyway) [I wish I could understand this more , when you said "leakage resistance of the 100nf cap" it confuse me more because I kept thinking - I don't have the smallest idea what is capacitance........anyway ...just the results....> if I touch the plates it change state from ON to OFF and if touch again from OFF to ON .....I left the circuit for minutes in one state and it remain until the plates are touch, because that is the expected behaviour I said it is OK. (by the way if I left the fingers on the touch plates the circuit oscillate, which might be interested to used in other situations. Again thanks a lot for your help.

[Now that I read again your post. Just for fun. Different behaviour. How do I make the circuit to turn ON only if I touch the plates.? Not a toggle switch. Just ON if I touch the plates.]

 

[ljcox]

I suggest you do an internet search for "capacitance"

You could also look at the data sheet for the 74C14 & note the input current.

Feel free to ask more questions if you need to.

I have attached a circuit that provides ON & OFF touch plates,

 

[prprog]

I suggest you do an internet search for "capacitance"

You could also look at the data sheet for the 74C14 & note the input current.

Feel free to ask more questions if you need to.

I have attached a circuit that provides ON & OFF touch plates,

ljcox - Thanks a lot.