Clap switch diagram, is this wrong?

Thread Starter

Xalendarian

Joined Apr 14, 2017
4
Hi all,
I've found online a diagram for a clap switch that can turn on more than one device. It's posted in more than one site, but I think there are some problem, may someone confirm or deny that?
When Mic is idle (no sound) tension is 3.15V and drop to 2.2V when blown on. At the base of BC548 is applyed a tension of 0.59V when mic is idle and -0.7V when i blown on microphone. I just think that is not the way this transistor can work, but well maybe i'm wrong. Can someone help me with this?

What I found strange is that if you search 9way clap switch this image appear really everywhere, and if is not correct I want to make it function correctly.
Thanks all guys!
9-Way clap switch - Copia.gif
 

ericgibbs

Joined Jan 29, 2010
13,274
hi X,
Does the circuit respond correctly when you 'clap' you hands near the mic.??

Blowing is not the same signal as a sharp clap.

E
 

Thread Starter

Xalendarian

Joined Apr 14, 2017
4
Well, I think my digital multimeter is too slow to detect a V change in only a clap :(. But every sound I make in the mic make the voltage lower, so the clap should make V lower to.
At the base of transistor the top V is 0.59V and every sound make this V go lower. Isn't that a really wrong way to pilot a BC548? I have the circuit mounted on braedboard so I can do all test you guys ask!
btw thanks for reply!
 

ericgibbs

Joined Jan 29, 2010
13,274
hi,
The IC1 555 pin #2 requires a low going pulse, from say Vsupply to 0v in order to be triggered.
A sharp sound ie: clap will produce the pulse, a 'blowing' sound has not got a 'fast' rising sound level, that is needed to produce a pulse.

Note the C1 capacitor makes the T1 Base an AC coupled circuit.

Do you follow OK.?

VR1 sets the 'sensitivity' of the sound level required.

E
 

Alec_t

Joined Sep 17, 2013
12,160
At the base of BC548 is applyed a tension of 0.59V when mic is idle and -0.7V when i blown on microphone. I just think that is not the way this transistor can work, but well maybe i'm wrong. Can someone help me with this?
When the mic picks up sound it generates an AC voltage. This causes C1 to charge and discharge alternately. However, the discharge current via R4 and T1 base-emitter junction is greater than the charge current via R3 and the mic capsule, so the average voltage on T1 base reduces, as you found. It is the positive-going peaks, though, which are needed by this circuit to produce, via T1 collector, negative pulses to trigger the 555.
 
Last edited:

RichardO

Joined May 4, 2013
2,271
I see several problems with the circuit.
1) There are no power supply bypass capacitors. there should be a 10 uF electrolytic cap where power comes into the circuit and a 0.1 uF ceramic cap on the power pins of each IC.

2) The 6 volt power supply may eventually damage the 7490. I would put a 1N4001 rectifier diode between the 6 volts and the circuit. Connect the anode to the 6 volts.

3) There are no back EMF diodes on the relay coils. The back EMF will damage the driver transistors. Add a 1N4001 across every relay coil with the cathode to power supply connection.

4) the pull down resistor on pins 2 and 3 of the 7490 is too high a value. It should be about 220 ohms.

5) there shbould be a resistor in series with VR1 to limit the transistor current with the poet set to zero ohms.


With this many obvious errors in the design, I would not expect the circuit will ever work. :eek:
 

ian field

Joined Oct 27, 2012
6,539
Replace Vr1 for a 100K resistor.
IMO: that's probably a bit on the high side - but it can't be any worse than the 1k pot with no series resistor to limit maximum collector current.

At its maximum 1k, its probably way too low.

I'd be thinking somewhere along the lines of 10 - 22k or so.

The transistor might also work better with a self compensating B/C bias resistor. Depending on where its gain spread landed - the operating point could be anywhere between the supply rails. A germanium transistor probably would've gone straight into thermal runaway.
 

Thread Starter

Xalendarian

Joined Apr 14, 2017
4
Yeah the problem is that actually it does not work, at least not correctly. thanks all for the reply, i'll try to fix this! IF (and it's a big one) I can fix, I will make a good diagram and post it so maybe no other attempt this one
 

ian field

Joined Oct 27, 2012
6,539
Try this modification to the transistor with a 100K base bias, and 10K collector resistor. Also try changing R1 on the 555 to a 100K.


View attachment 124781
The quick & dirty method is to guesstimate what you think the collector resistor should be and use a 1M pot to adjust the C/B resistor for Vcc/2 at the collector. Its worth shorting the other end of the input coupling capacitor to GND so hum pickup doesn't skew the adjustment.

Then you just measure the pot and fit the nearest preferred value.
 
Top