Can someone please help me with this circuit - Page 1
Forum section: electronics / beginners

author: uk.casmith
author_rank: Contributor
timestamp: April 29, 2019, 12:44:54 pm
content: Can someone please help me with this circuit.The base of the transistor is connected to a 555 timer.Please could someone talk me through what happens when the 555 timer fires and when it doesn't fire.Why do we need a voltage divider when we are using a digital circuit? I thought we only used voltage dividers with transistors to stop the signal from clipping below 0volts.I've been stuck on this for awhile; please could someone walk me through what is happening here in as easy to follow language.Thanks in advance for your time,Cameron.

author: ArthurDent
author_rank: Super Contributor
timestamp: April 29, 2019, 12:55:26 pm
content: The resistors set the base voltage on the transistor at about 1/11 of the Vcc so if you have a supply of 5 VDC then the base is at about 0.45 volts and the transistor will not conduct. When the 555 fires, there is a pulse sent to the base through the cap that raises the base voltage and the transistor conducts until the charges equalize on the cap.

author: uk.casmith
author_rank: Contributor
timestamp: April 29, 2019, 01:01:48 pm
content: Why do we need to set the base voltage? Surly when the 555 timer fires this will be enougth to open the transitor and close it again? Why add that extra complication?

author: uk.casmith
author_rank: Contributor
timestamp: April 29, 2019, 01:14:08 pm
content: Just re-read what I put and thought I sounded a bit mean. Sorry about that please read it in a cheerful way

author: ArthurDent
author_rank: Super Contributor
timestamp: April 29, 2019, 01:22:38 pm
content: Sounded like a question to me.Besides being good practice to set the voltage on the base rather than let it float, the coupling cap is quite large so the 100? Mfd electrolytic and the 1K resistor form a R-C network that give a time delay so the base voltage stays high when the 555 triggers and the transistor will conduct until the voltage on the base decays below the point where the transistor conducts, maybe around .6 volts.

author: rstofer
author_rank: Super Contributor
timestamp: April 29, 2019, 05:48:00 pm
content: 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.

author: Jwillis
author_rank: Super Contributor
timestamp: April 29, 2019, 06:28:56 pm
content: 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.

author: floobydust
author_rank: Super Contributor
timestamp: April 29, 2019, 08:00:27 pm
content: 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.

author: frobet
author_rank: Newbie
timestamp: April 29, 2019, 08:26:41 pm
content: You should read carefully before assuming. Also check the image he posted, then you will understand what he is trying to build.

author: lordvader88
author_rank: Frequent Contributor
timestamp: April 29, 2019, 09:01:55 pm
content: 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.