for this circuit:
http://i59.tinypic.com/2ykh0kn.jpg
how do you calculate how much current is going to come out of the put to the loudspeaker? i know once the voltage threshold is met there is basicly a surge out of the cathode. (correct me if im wrong)
also how do you know how much resistance that the loudspeaker needs?
I'm not sure how an scr is driven like that..
But to measure the speaker, use a multimeter measure the resistance, thus how many ohms it is...
Edit...
Ooh it's not an scr! Sorry kinda distracted by the moderator gunnng to remove me 
Anchorman44:
For this circuit, how do you calculate how much current is going to come out of the put to the loudspeaker?
Precious little current. At 6V supply, the capacitor charges to about 3V before the PUT is triggered, the PUT drops about a volt of this so you have a sawtooth output of about 2V. Over 108 ohms (100 ohm resistor plus 8 ohm speaker), that amounts to 2000/108 or 18 milliamps.
Anchorman44:
Also how do you know how much resistance that the loudspeaker needs?
"Needs" is not meaningful. Eight ohm loudspeakers are common, but if you are going to put a resistor series to make it up to 100 ohms or so, it is better to use as high a speaker impedance as possible, preferably 50 or so as this then becomes a greater proportion of the total resistance. If you could find a 100 ohm speaker, you would need no extra resistor at all.
The greyed-out part of the circuit describes a terrible way to flash an LED. I cannot discern form where you got that picture, but many "instructables" and similar descriptions (such as YouTube) on the Internet are shockingly bad or even dangerous practice.
Hi, by the looks of the picture, its from a book of circuits, or design ideas, and there is a layout diagram as well, I would hope that they have given you a parts list as well, that will tell you what resistance the loudspeaker is.
I think you want to know how loud the speaker will be, that depends on the physical size of the speaker, its resonant frequency range and the frequency that the oscillator is designed to run at.
Tom......
cjdelphi:
I'm not sure how an scr is driven like that..But to measure the speaker, use a multimeter measure the resistance, thus how many ohms it is...
Edit...
Ooh it's not an scr! Sorry kinda distracted by the moderator gunnng to remove me
An easy mistake to make - the gate is on the cathode for an SCR, anode for a
programmable unijunction transistor. But no-one really uses them anyway so its
understandable.
A circuit that old is relying on the probably rather high ESR of that 2.2uF capacitor to not damage the LED. Or not so quickly that it immediately fails.
According to the datasheet, the forward voltage is more like 1V. However, you can't just use the numbers to calculate power, because that is an impulse, a capacitive discharge into the speaker.
TomGeorge:
Hi, by the looks of the picture, its from a book of circuits, or design ideas, and there is a layout diagram as well, I would hope that they have given you a parts list as well, that will tell you what resistance the loudspeaker is.I think you want to know how loud the speaker will be, that depends on the physical size of the speaker, its resonant frequency range and the frequency that the oscillator is designed to run at.
Tom......
its a book called Make : Electronics. It seemed like a great book but this is the second time that i have posted a circuit from it on this forum and both times the circuit was said to have bad design. So If you can offer me any other books that helped you learn I would be grateful for any help!
Paul__B:
The greyed-out part of the circuit describes a terrible way to flash an LED. I cannot discern form where you got that picture, but many "instructables" and similar descriptions (such as YouTube) on the Internet are shockingly bad or even dangerous practice.
How would you design an LED flasher circuit?
polymorph:
A circuit that old is relying on the probably rather high ESR of that 2.2uF capacitor to not damage the LED. Or not so quickly that it immediately fails.According to the datasheet, the forward voltage is more like 1V. However, you can't just use the numbers to calculate power, because that is an impulse, a capacitive discharge into the speaker.
Umm.. I understand what you mean by an impulse. But how would you calculate the power then if you couldn’t use numbers? Trial and error?
its a book called Make : Electronics. It seemed like a great book but this is the second time that i have posted a circuit from it on this forum and both times the circuit was said to have bad design.
Wow, that is a fairly new book, to have such an old part in such an old circuit in it. Sadly, there is a lot of that going around. A Kickstarter project just concluded that, although I am unable to review it, has not impressed me with the few excerpts I was able to see.
Umm.. I understand what you mean by an impulse. But how would you calculate the power then if you couldn’t use numbers? Trial and error?
Hm. I suppose I'd use the RC discharge calculations into 108 ohms (assuming that is an 8 ohm speaker) over about 3 Tau (time constants, one time constant aka Tau = RxC) and er... I think we're getting into calculus.
The question is, is it loud enough for you? If you really need this to be a certain power to the speaker, dump this crude circuit and use something different.
I really don't care for the way they draw schematics in that book. Nonstandard and not clear.
Anchorman44:
How would you design an LED flasher circuit?
Use an LM3909? Designed especially for the purpose.
Unfortunately, now priced as collectors items! 
I know you didn't draw that, Paul, so this isn't a criticism directed at you... why are there so many badly drawn schematics out there?
polymorph:
I know you didn't draw that, Paul, so this isn't a criticism directed at you... why are there so many badly drawn schematics out there?
Why is there an Internet?
{I just chose an illustration - from a thousand or so perhaps - that was properly legible, would render and had the minimum essential details.}
I last used PUTs in 1969 and that circuit looks typical of what I used to do with them. I was working for an industrial electronics company at the time and we had no problems with discharge current. The thing is that LEDs are quite resilient for short low reparation rate circuits and can often be safely driven at 10 to 50 times the continuous current rate. I do not think that circuit is so very bad.
If all it is you want to do is flash an led..
This is the cheapest (? Maybe component wise) circuit.. I'm just wondering if the 555 is a cheaper ic, however the 555 requires more external components.
The 555 requires one more component, a second resistor. Although you can also wire up a 555 with only one resistor and one capacitor.
Hi, if you are not worried about flashing frequency, then buy a flashing LED, all it needs is a resistor.
Tom....
I think he's trying to learn electronics.
Blinking LEDs don't usually need a resistor.
TomGeorge:
Hi, if you are not worried about flashing frequency, then buy a flashing LED, all it needs is a resistor.Tom....
I'm trying to learn electronics as a whole. I'm interested though - what new parts replace PUTs?
I've never used a 555
what new parts replace PUTs?
PUT stands for Programmable Unijunction Transistor, which was a development of the Unijunction Transistor.
At first there were a range of Unijunction Transistor with different zeta or breakdown voltages. Then the PUT allowed you to set that value by the ratio of two high value resistors. Their main application was in commutating ( turning off ) SCRs in low voltage control circuits. They were also used in missing pulse detectors. The relaxation oscillator that you are looking at was an interesting alternative use.
SCRs were used because they had a much higher current capability and lower dissipation than transistors did at the time. Transistors then developed a much higher current capability and the FET later developed a much lower dissipation.
Then integrated circuits came along and the NE555 was one of the most flexible devices and could be used not only like a PUT but also as a monostable and modulator. Integrated circuits were perhaps the greatest legacy of the "Space Race" for the Moon.
ICs like the 555 timer are -much- more flexible than the PUT. Integrated Circuits really advanced electronics well beyond the stage of old computers, where one rather large PCB would have one or two logic gates built from discrete transistors, resistors, diodes, and capacitors.
