| Electronics > Beginners |
| Can someone please help me with this circuit |
| << < (2/2) |
| rstofer:
Perhaps the question should be: Why is there a cap in the first place? This is a really good question! The cap allows some kind of AC signal (riding on DC?) to trigger the transistor. It also places a time constraint on the width of the pulse but it is so large that this won't be a factor. If neither of those conditions matter, why have either? You can put some kind of current limiting resistor (470 Ohm?) in series between the 555 output and the transistor base. Then again, why have a transistor? The 555 can drive some load current. Not as much as the 2N2222A though. What is the load? It seems like it might be a speaker? If so, what is this circuit supposed to do? It certainly looks like some kind of amplifier (badly biased) but, if so, the diode is a little odd. I think we need more information. |
| Jwillis:
The astable output of the 555 is equivalent to a steady DC of about ½ the voltage source combined with a square wave AC (audio) signal. The capacitor blocks the DC, but allows the AC to pass.Now consider a DC voltage on a speaker and what it does to the speaker.It would keep the coil in the speaker charged to the value of the DC Voltage for Pin 3 .This distorts the AC signal in the speaker at the vary least or can destroy the speaker if the voltage is high enough. |
| floobydust:
It does appear to be a silly circuit for a few reasons. It's supposed to be switching on/off yet it has bias resistors for Q1, and a coupling capacitor C7. That's what you'd have in a linear amplifier, it's confusing. I think C7 is there to ensure Q1 is not always on, which would roast the speaker. The 555 output might be high at idle? Have to see the rest of the 555 circuit to see why C7 is even needed. C7 limits the maximum on-time pulse width for Q1. But because the 555 drive goes through big C7, and the E-B junction of Q1 is like a diode, the capacitor will eventually charge up with net DC. It also looks like the wrong polarity (- to 555 timer out). I don't know the frequency and duty cycle here. When the 555 timer output is +ve, C7 charges through the E-B junction and R3. This turns on Q1 and the base pulses up to around +1V. When the 555 timer output is GND, C7 discharges through R2 and R3 and takes Q2 base negative to -4.5V. OP I think the circuit is not so great and could be done differently, depending on the rest of the 555's use. |
| frobet:
You should read carefully before assuming. Also check the image he posted, then you will understand what he is trying to build. |
| lordvader88:
The 555 could be directly connected to the base, with nothing else connected to the base. But in that case, the you would blow up the transistor, as soon as the output goes over about 5V and you have +5V =Vbe. Also it will feed a lot of current, So unless some chip only puts out the exact base voltage/current u want, you have to set it up with other parts to keep it operating like u want. |
| Navigation |
| Message Index |
| Previous page |