Here is precisely what I made up on the breadboard:

I can successfully use the transistor (BC557B) to switch on/off a LED, so I know the transistor is working.

Sorry for maybe making obvious mistakes.

Thanks guys.

 

The BC557 is a PNP transistor. The emitter needs to be more positive than the collector.

You must bring the base within 0.5v of the emitter to turn it off. You must bring the base to 0v to get it to sink current.

What you really need is an NPN transistor, and you will also need a base current limiting resistor. Anything from 1k to 10k will work in this case.

 

Yep, you need an NPN. Like the BC547 for example. Since you already have R3 R4 and C1, remove the 47k`s and the C4. Put the diode from trigger to Vcc.

(You had put two times the same circuit in series.)

Tell us what NPN transistor you have at hand.

 

This is being a wonderful learning experience for me. I am 4 years into my studies to become a physicist/computer scientist and this really put some "everyday life" on top of my solid state physics courses http://www.allaboutcircuits.com/vol_3/chpt_4/1.html is really good reading.

First of all I have the BC547B (NPN) transistor at hand also.

This is the new revised circuit based upon your previous 2-3 replies:

This green part of the circuit is "based" on praondevou's trigger-circuit proposal. The all black part of the circuit is the 555 monostable test circuit from Bill Marsden's writeup (http://forum.allaboutcircuits.com/showthread.php?t=14018).

Finally the blue part is what I stripped from the wireless doorbell. Basically a 3V voltage supply (2x 1.5V batteries) which originally led to a LED doorbell. It made the LED light up for as long as the doorbell sounded, which is about 5 seconds. The 1.6-1.8V I originally reported this signal to be was with the LED connected, now it is disconnected it i get 3V for 5 sec. This power source is switched on/off by the wireless transmitter indicated as this unknown/wireless component C

I've thrown in some red roman numerals to mark certain junctions in the circiut - just in case someone needed to refer to one of the junctions.

I have been a bit in daubt about what to do with the negative-pole of the doorbell 3V power source. But I figure it best be connected to the negative pole of the 9V battery (grounded). Is this right?

Does the above circuit look like something which would work?

Thanks again
Wuhtzu

 

Connect D2 from pin2 of the 555 to Vcc, not from the collector. The diode avoids the voltage on the trigger input going much higher than Vcc.

If the circuit doesn't work you may try to increase the 10k's a little bit and decrease the 100n. But try it first the way you drew it.

Connect the two minus' from the batteries together.

 

 

I have to ask this question which is obviously stupid, but I simply do not get what that 1N4148 diode is doing/helping in parallel to R3 and R4. Current can still run through R3 and/or R4 to pin 2. Having a diode there seems like having no connection at all.

 

if you look at the post above mine?the question has been answered........

 

I have to ask this question which is obviously stupid, but I simply do not get what that 1N4148 diode is doing/helping in parallel to R3 and R4. Current can still run through R3 and/or R4 to pin 2. Having a diode there seems like having no connection at all.

1. Transistor is OFF. the capacitor is discharged (same voltage level on both sides)
2. Transistor is being turned ON. The left side of the capacitor is pulled to ground, the right side also in the first moment, then the right side of the capacitor is being recharged to Vcc via R4. The capacitor is now charged (right side positive.
3. The transistor is being turned OFF. The left side is pulled up to Vcc via R3. The right side of the capacitor could be pulled to a higher value than Vcc which may not be good for the 555. The diode would clamp that voltage to Vcc + Vfd (forward drop).

It's just a measure to avoid possible problems. It doesn't mean that will absolutely not work without the diode.

 

I will try this design tomorrow sunday, or monday and hopefully not have any further questions because it hopefully just works

 

Thank you all so much.

It works perfectly now.

It all boiled down to my lack of knowledge about transistors. If I had chosen the right transistor type (npn) and oriented it correctly it would have worked the first evening I got the "trigger it by a transistor" idea

I will post my complete circuit as soon as I'm all done.

 

I have a followup problem/question.

The circuit (wired up on a bread board) triggers on almost anything. Knocking in the table next to the breadboard, tapping (moderately hard) on the board with your finger, plugging an floating wire (connected to nothing) into ground etc.

What could I do to minimize this behaviour?

Or do you think it's just a crappy bread board? (I plan on making a pcb for the circuit once it works).

Thanks
Wuhtzu

 

Breadboards can be finicky (unreliable) after they've been used a lot, or if someone jammed parts with big leads (TO-220 packages) or large wires into the sockets.

They can also accumulate metal shavings or bits of wire in the board itself, particularly if someone used stranded wire instead of the proper solid wire. If this happens, you might be able to save the board by removing the strips from the bottom, cleaning any mess out and straightening the contacts, then reassembling it. You could also just try a different area of the breadboard.

Other than that, toss it in the bin and start with a new one.