Touch Switch

[Raq23]

I am implementing a touch switch: when i touch the metal plate it triggers the 555 in the monostable mode, that goes into a 7476 (JK Flip Flop)which simple toggles, it sends the high into the relay and in turn lights up an LED connected to the normally open switch of the relay (relay: 5v - 1am). But my problem arises when I touch the metal plate again to turn off the LED. The LED turns of for a second but then turns on again, I'm thinking there is still some voltage within the coil of the relay. I placed a diode parallel to the relay coils (reversing the polarity of the diode) to avoid any voltage from going back through the coil but it doesn't seem to help much. Any help?

 

[boxxy]

I don't know the 555 timers - I have always tried to avoid them because of problems like these.

All I can suggest is that you simplify your circuit with a D type flip flop (4013) configured with the Q o/p wired back to the data input. A low going edge at the clock input driven from your touch switch circuit will clock the value on the data input to the Q output and set up the data input for your next press. You don't need a relay to drive a LED - the D type output can sink 10mA. If you need more use a transistor, lot cheaper than a relay and no horrible inductive coils to worry about.

Rgds, Boxxy

 

[Raq23]

Thanks Boxxy for the response. I'm working with a Snubber Circuit hopefully that will work. I just have to figure out what resistor and capacitor to use and whether a diode should also be included.

 

[richs]

Hi-

Signetics 555 timer ap note AN170 has this rather criptic
but useful statement:

"6. Double triggering of the TTL loads sometimes occurs. Why?
Answer: Due to the high current capability and fast rise and
fall times of the output, a totem-pole structure different
from the TTL classical structure was used. Near TTL threshold this output exhibits a crossover distortion which may double trigger logic. A 1000pF capacitor from the output to ground will eliminate false triggering."

Hope this helps.

Cheers,

Rich S.

 

[BrianR]

As richs said, the totem pole structure of the 555 output has some overlap during switching when both transistors are on and you can have a very brief pulse of up to 1A flow. Appropriate capacitors at the supply pins of the 555 help but another solution is to use a CMOS 7555 which does not suffer from this problem.

 

[rodar]

Make sure you have pin 5 bypassed to ground with a
cap in the 0.1uF range. This is an internal reference
level.
Rodar

 

[Raq23]

Thanks all, I worked perfectly after I placed a 100uF capacitor at the + and - supply of the 555 and the circuit works great. The turns the lamp on and off without any gliches.

 

[fjrg76]

Have you already tried just a single D-latch?? Besides there is a TTL chip that performs the same task as 555, this is the 74LS221

 

[60hzhmmm]

Hi raq23: I don't know if you're doing a one-up or not but I caution you on touch switches that work thru mechanical relays.. There are lots of gotchas lerking about. As a kid I once made one. I gave it to a girl friend she loved it till she leaned a metal milk crate next to it and it decided to switch, a lot. A whole lot. Melting into a pile of slag. You might concider the many IC's designed specifically to do just/only the touch function for example:

http://www.qprox.com/downloads/manuals/e116.pdf