Simple proximity detector (not light dependent)

[teej212]

http://www.youtube.com/watch?v=NTIWI94KRg8

i found this while researching transistors used as a switch.  heres how the video uploader described the circuit :

It's very simple.First and second transistors are little amps who "boost" all electricity around, and they are "open" third transistor connected on LED. These three transistors are conected in triple darlington. componests I used are:
1x1MOhm
1x100kOhm
1x330Ohm
1XLED
3xBC547 NPN Transistor (or 2n2222)
This is how it looks one simple transistor circuit whose VERY sensitive and can detect human electricity and can detect mains cables in a wall!

can anyone make sens out of that? where do the resistors go, i thought a triple darlington would be emitter of the first transistor feeds into the base of the next one, and so on with all collectors going to +V. any ideas, i think this would be a great substitute for the much more complex circuit that was posted on the wah probe work alike on another DIY forum.  as a plus, the guy who made the video says its very sensitive, which could even be toned down if need be. 

[earthtonesaudio]

That IS very sensitive.  The antenna is barely anything!  I'm impressed.

That description is confusing though.  Either it's a triple darlington or it's a pair of "little amps" and a third transistor, but I don't see how it can be both of those things.

[petemoore]

  HArd to assume much with so little.
  so..
  What's holding the base/input where it is loose it's able to be pulled on ?
  1meg ?
  What's teh 100k doing ?
  Inbetweener bias resisitor?
  330ohm emitter thing on the end transistor I dunno.
  Just keep making gain until it's high enough that the input can be called 'sensative' and starts triggering when hand is near.
  I'm just guessuming. Seems feasible that plain 'ol hum could be used to bring a base up to where an output says 'that's enough hum I could light an LED or something !'.

[The Tone God]

This is probably a touch circuit that senses the AC picked up by the body and amplified. Yeah that video seems impressive but that is probably because they played around to find the most range in that current setting before they shot the video. The problem with that style of circuit is the need to be near AC, usually power lines, for it to work and depending on your distance from the AC source the range of sensitivity and output will change so this circuit is woefully unrealiable. Generally this style of circuit is used in things like cheap lamp touch switch as they are easy to build and the line AC in the lamp can be used at the AC source but this is just as a switch and not as a CV output.

I would not use this style of circuit for what you are thinking about.

Andrew

[deadastronaut]

these are great...until you put your shoes on...

tried a few prox sensors....

[earthtonesaudio]

This is probably a touch circuit that senses the AC picked up by the body and amplified. Yeah that video seems impressive but that is probably because they played around to find the most range in that current setting before they shot the video. The problem with that style of circuit is the need to be near AC, usually power lines, for it to work and depending on your distance from the AC source the range of sensitivity and output will change so this circuit is woefully unrealiable. Generally this style of circuit is used in things like cheap lamp touch switch as they are easy to build and the line AC in the lamp can be used at the AC source but this is just as a switch and not as a CV output.

I would not use this style of circuit for what you are thinking about.

Andrew

...Unless you use an AC adapter for the pedal. 

[teej212]

well i shot the guy a PM, hopefully he answers.