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Mains hum 555 timer circuit problems

I found a circuit in an old book of 555 timer projects for a non-latching mains hum single plate touch switch, but I just can't get it to work!

The touch plate is connected to pin 2 of the 555 via a 0.1uF capacitor.

Pin 2 is also connected to the cathode of an IN4001 diode, and the anode of that diode is connected to 0v

Pin 2 is also connected to the anode of a second IN4001 diode, and cathode of that diode connects to Vcc. A resistor which supposedly sets the sensitivity of the circuit is connected in parallel with this diode between pin 2 of the 555 and Vcc - suggested values "in the range 47K-10M"

According to the book, the 555 should trigger permanently when the resistor value gets as high as 10M. It doesn't, nor at 20M, nor 30M. This suggest to me there is a flaw in the design.

I can get it to work with some stability (but not enough!) by simply connecting pin 2 to the touch plate via a longish wire. I suspect this is because the wire is picking up background noise sufficient to stop the 555 triggering, and my finger is then dragging the signal down a bit, and the

555 then triggers.

I can also get it to work by going for a leakage based (i.e. two contact pad) solution of connecting pin 2 to Vcc via a 10M resistor and one pad to pin 2 and a second to 0v - but it's not that sensitive + I really could do with a one-pad solution.

Other info about the circuit:

Supply is a 9v battery. There is no indication the circuit needs to be grounded. Pin 6/7 are connected to 0V via a 1uF capacitor, and to Vcc via an 820K. Pin 5 connects to 0v via a 01.uF capacitor. I'm operating it in the house where there should be plenty of mains hum to drive the circuit Pin 3 drives a 9V relay via another IN2001 diode (another IN4001 diode + 1uF capacitor sit across the coil of the relay)

So if anyone has:

  1. Any ideas for getting this circuit to work properly and reliably OR
  2. A really good proven one pad non-latching circuit design which can preferably run from battery and doesn't need to be grounded OR
  3. Any other proven one-pad non-latching design. I won't rule out a solution that needs to be grounded and/or mains powered., but its just not as convenient.

TIA.

Midge.

Reply to
Midge
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Replace the 1Nxxx diodes with 1N4148. Use a CMOS 555

Reply to
GPG

hi, dont worry ... i have designed a circuit on 555 timer send mail kaushik899 @ gmail .com i will send .

kaushik

Reply to
cozz in or out 

--
Try this: (View in Courier)

                                  
+9V>-------------+------+-------+---------+------+
                 |      |K      |         |8     |
                 |   [1N4148] [820K]  +---+---+  |
                 |      |       |    2|_ Vcc _|3 |
IN>---[0.1µF]--[1M] OUT TO RELAY
                 |      |K      |    6|       |  |
                 |   [1N4148]   +-----|TH     |  |
                 |      |       |    7|_     _|4 |
                 |      |       +----O|D     R|O-+
                 |      |       |     |  GND  |
                 |      |       |+    +---+---+
                 |      |     [1µF]       |1
                 |      |       |         |
                 |      |       |         |
GND>-------------+------+-------+---------+ 


With nothing on IN, rotate the 1 megohm pot until OUT goes high,
then back the pot off until OUT goes low.  

Now touch IN, and OUT should go high for as long as you keep
touching IN.

If the sensitivity is too high, back off R1 until you get the
sensitivity you need.

BTW, how reliable does the circuit have to be?
Reply to
John Fields

I have a circuit just like that. Mine is a "lightening detector" (spark) detector. It has a pot to set the sensitivity and a single common npn transistor to amplify the signal into pin 2. (pulse is capacitively coupled into pin two like it shows in the application notes - 10 K pullup on 2 with a .1 ufd cap to take the negative transition into it).

2N2222A has a 10K pull up on its collector and the base goes to a wire that detects the static with a crude high pass filter, since I don't really want it triggering on hum. If I touch the antenna it triggers very reliably, so I'm thinking if you add an npn transistor to amplify the touch plate and use AC coupling for the trigger you should have what you want.

My device also triggers with one of those piezo electric igniter used for lighting propane grills from about four feet away. Antenna is a

1' piece of wire. 
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Reply to
Brendan Gillatt

Just a bit of background.....

I'm setting up the switches to help someone who is nearly paralysed with MS to control a few things around her - hense the need for a one pad solution for ease of use.

Switching does need to be pretty reliable so I'll try some of your ideas and see how I get on.

Thanks for the prompt help.

Midge.

"Brendan Gillatt" wrote in message news: snipped-for-privacy@pipex.net...

>
Reply to
Midge

hey midge ..did my circuit helped you .... ???

Reply to
cozz in or out

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