Unexpected behavior from 555 circuit - Page 1
Forum section: electronics / projects

author: rexxar
author_rank: Frequent Contributor
timestamp: January 04, 2014, 06:10:49 pm
content: Hi all, I have a problem with this 555 circuit that has me banging my head into the wall.It's a simple 555 buzzer and a one transistor amplifier. The idea is you have one pot to control pitch, and another one for volume. Thing is, when you turn the volume pot, the pitch changes. I put the scope on it, and it looks sort of like the output of the 555 is getting loaded down. And instead of a nice square wave output on the transistor, I get some really distorted waveform. I'm not even sure what to call it, I'll upload a picture.I have no idea what I'm doing wrong. I've built this circuit before and had it work. In fact, I still have the first one working on a board here, and I've copied the circuit from that. I've rebuilt it on the breadboard about four times from new parts, but still no.I'll upload a schematic (sorry if it's hard to read, I'm still getting the hang of DipTrace), hopefully someone else will see a glaring error and smack me over the head.

author: kfitch42
author_rank: Frequent Contributor
timestamp: January 04, 2014, 07:00:12 pm
content: CAVEAT: I'm a newb.The first thing that strikes me is that the only resistance between VCC and ground when the transistor is on, is the speaker. I have been told, as a rule of thumb, to put resistors on all three legs of a transistor. Is the (8 ohms?) load from the speaker enough to drag down VCC? Possibly put VCC on the scope to see what is happening there.P.S. Debugging rule number 1: Thou shalt check voltagesP.P.S. Have you put it in LTSpice (or similar) to see what happens there?

author: rexxar
author_rank: Frequent Contributor
timestamp: January 04, 2014, 07:18:49 pm
content: I did have the idea to put it in LTSpice just after I posted, and it works as expected. Voltages are fine, there's a bit of noise (at the frequency of the buzzer) going back up VCC, but that's not concerning, I think.

author: Andy Watson
author_rank: Super Contributor
timestamp: January 04, 2014, 07:39:25 pm
content: Quote from: rexxar on January 04, 2014, 06:10:49 pmYes, and yes! You appear to be controlling the volume by shorting-out the output of the 555 with the 1k pot? It's not going to like that. Also, as kfitch42 has pointed out, if you're using a low-impedance speaker that will put a very heavy load on the power supply. Remember there will be a D.C. component to the waveform.At the very minimum you need to re-arrange the 1K pot so that it doesn't work by shorting-out the 555. Secondly, you probably ought to have some means of blocking the D.C. to the speaker.

author: fcb
author_rank: Super Contributor
timestamp: January 04, 2014, 07:41:07 pm
content: Watch out that your 500K doesn't go too low - pin7 is a transistor to gnd - probably put a resistor (a couple of K) in series with it.

author: rexxar
author_rank: Frequent Contributor
timestamp: January 04, 2014, 07:55:54 pm
content: Quote from: Andy Watson on January 04, 2014, 07:39:25 pmHm, that's a good idea. Would a 1pf ceramic capacitor do?QuoteI thought about that, but it worked the first time I tried it, so I just went with it. Just plugging values into SPICE, fixed 470R to ground, and the 1k pot to VCC seems to work ok. I'll probably put something like 330R in series to limit it, does that sound like a good idea?EDIT: I got it working. I switched the bias resistor and pot, and added a 3.3k resistor between the collector and Vcc, to limit current. I don't know why it worked the first time, but after staring at it for hours, at this point I'll take 'magic' as an answer.Now I'm off to Mouser because I murdered three speakers in the process

author: grumpydoc
author_rank: Super Contributor
timestamp: January 05, 2014, 08:37:48 pm
content: QuoteItwashard to read, I'm afraid. It's not just you but a lot of the schematics posted seem to be poorly drawn which makes understanding what's going on much harder. I'm not sure it's always "newbies" either.Laying out schematics is a bit of an art and you'll get better at it as you go along. One thing to do is to look at decently drawn schematics and try to emulate what's there.I'm no expert at it by a long shot but a couple of rules of thumb might help:Try to use a single horizontal line to depict the power rails or ground.Minimise crossing linesIf there is a signal flowing try to adopt the convention of inputs on the left outputs on the rightIf you have a lot of ICs which take Vcc and ground it's usual to omit those on the main circuit as they can clutter things. Show on a separate sheet with decoupling.Don't cram too much on one sheet - or too littleNone of these are absolutes by any means.I'd re-draw your original circuit as follows.Which I find easier to follow.The fact that you're shorting the 555 output with the 1k pot now become more obvious. The fact that there's a 10u cap doesn't really alter that - the impedance of 10uF at 1kHz is only 15 ohms.As has been pointed out there is nothing to limit the current flowing in the 2N2222. Disregarding the 555 output for a second if you have the volume pot turned to give 1kohm and 9V on the Vcc rail then there will be about 240uA of base current in the 2N2222 so about 12mA in the collector-emmitter circuit (i.e through the speaker) which isn't so bad. However whatever output the 555 can manage will increase the base current, 555's can manage up to about 200mA so driving into 1k most of that can go into the base. You'll get about 1A flowing through the speaker with ease.As you turn the pot towards 0 ohms the voltage divider formed by it and the 10k resistor will eventually mean that the transistor will be biased off unless the 555 output pushes the base voltage above 0.6V. You won't have a volume control though because the (fairly) sharp edges from the 555 will mean that the transistor is still pretty much either on or off and when on in saturation - that is until the 555 can't drive the ever lowering resistance enough to make the transistor turn on. At some point it sounds like the load on the 555 is changing the frequency.QuoteWhat, as a DC blocker to the 'speaker - only if your hearing stretches up to MHzQuoteNot much of a learning exercise then - you don't know why it didn't work and you don't know why what you did to fix it, fixed it. I'm not getting at you - I've had projects like that tooHowever it's not the best way to get useful experience.I'd suggest having a look at the common transistor amplifier configurations - common emitter, common collector and common base. Then have another think about your circuit.