# 555 timer

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#### luxuchenghu

##### New Member
We know we can easily build a square wave generator using 555 timer, my professor say, the frequency of it can hardly be higher than 1mhz. Who can tell the reason. Base on: f=1.44/(r1+2*r2)*c, I think any frequency is avaliable. Does it because we can hardly find the right r and c to build some too high or too low frequency or maybe it will be very unstable in the high frequency? So what is the frequency range we usually use 555 timer to generate? Can we use 555 to generate frequency below 20hz?

Thank you

#### colin55

##### Well-Known Member
It goes from about 200kHz to less than 1Hz and then you can get time-delays from 1 second to an hour or more. That's why they are also called "timers" due to the long time-delay they can produce.
Look at the data sheets for all the different types of 555's and the frequencies, delay times etc.

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##### Banned
The analog components in most 555's simply can't handle the frequency lux. In theory sure it works, but these are real devices not math equations at work here =) I'm sure there are some higher speed 555's out there, but once you get to that point it's usually smarter to use a proper oscillator.

#### luxuchenghu

##### New Member
I see, so you mean we call it timer, because its frequency can be much much more lower than 1hz. Thank you very much, you really helped a lot!!!

#### luxuchenghu

##### New Member
I see, much appreciate.

##### Banned
Apparently I miss read your post please ignore what I said previously =P I was thinking higher frequencies not lower.
Yes you can do lower frequencies than 1hz. Much lower. I think the longest timer I've seen a 555 used for is about 10 minutes, anything more than a minute and the component variance might influence your timer so it's not really good for high precision timeing.

#### chingyg

##### New Member
Apparently I miss read your post please ignore what I said previously =P I was thinking higher frequencies not lower.
Yes you can do lower frequencies than 1hz. Much lower. I think the longest timer I've seen a 555 used for is about 10 minutes, anything more than a minute and the component variance might influence your timer so it's not really good for high precision timeing.
Ya, thats true, no precision timing. I learnt it the hard way. I was using a 555 for the detonator of my fuel air explosive. Its got 5 liter of petrol and a 50g ANAL charge in the middle. I set it to 15 min,(cause I thought these things are nice and accurate and stuff.) and ran for cover. I waited and waited until like 20 minute have past. Then My friend and I thought something is wrong so we went to check, and when we were like 10m from that thing you just see a huge fire ball, as bright as the sun, we got thrown back a few meters and burnt our eyebrows. Haha. So listen to Sceadwian, don't use it for precision timing.

#### Thunderchild

##### New Member
Ya, thats true, no precision timing. I learnt it the hard way. I was using a 555 for the detonator of my fuel air explosive. Its got 5 liter of petrol and a 50g ANAL charge in the middle. I set it to 15 min,(cause I thought these things are nice and accurate and stuff.) and ran for cover. I waited and waited until like 20 minute have past. Then My friend and I thought something is wrong so we went to check, and when we were like 10m from that thing you just see a huge fire ball, as bright as the sun, we got thrown back a few meters and burnt our eyebrows. Haha. So listen to Sceadwian, don't use it for precision timing.
my post code is nn297hf plese let me know if you are ever withing 100 miles of this location and I'll get the hell outa here

#### Torben

##### Well-Known Member
Wow. *Please* say that you tested the thing with, say, an LED before attaching it to the actual device? Y'know, to make sure the timing was right, and reliable?

Torben

#### dougy83

##### Well-Known Member
... and a 50g ANAL charge in the middle.
Sounds quite uncomfortable.

#### chingyg

##### New Member
Wow. *Please* say that you tested the thing with, say, an LED before attaching it to the actual device? Y'know, to make sure the timing was right, and reliable?

Torben
Ya, tested with a buzzer, a few times, worked out the average accuracy to be 10%. that is if I set it to 15min, it has to go off at like 16.5min. god knows what happened. Maybe the variable resistor's scale ain't perfectly linear or something.

#### chingyg

##### New Member
my post code is nn297hf plese let me know if you are ever withing 100 miles of this location and I'll get the hell outa here
Dude it ain't that bad, I made another one of these fuel air recently. The fireball was barely 10m in diameter. so you only get the hell out of wherever you live when i'm 10m from there. and the chance of that happening is about 245ppt.
If you dont believe me here is the video.
View attachment Movie.zip

##### Banned
chingyg. Home made fuel air explosives? That takes balls =) Walking up to one after a misffire takes even bigger ones =O If you're messing with explosives at least use a remote detonator, that way you can remove the voltage source and safe the trigger by shorting the two leads together before you get close to it =\

#### chingyg

##### New Member
chingyg. Home made fuel air explosives? That takes balls =) Walking up to one after a misffire takes even bigger ones =O If you're messing with explosives at least use a remote detonator, that way you can remove the voltage source and safe the trigger by shorting the two leads together before you get close to it =\
Build a remote detonator, that take money, money I don't have. I used to have and old remote car wired to and SCR that is shorting a car battery across an 1ohm 0.25W resistor. Worked for a while then the remote car's circuit failed, cause I dropped a drop of solder on the board while I was working, shorted something and the thing don't wana work again.
I am planning on making a new one. What circuit should I use, what about a cellphone speaker driving an SCR that is discharging a 320uF 350V cap into an 100Ω Resistor? or do you have a better idea?

##### Banned
chin, you don't have 50 feet of wire and a 9 volt battery? I'm disappointed.

#### chingyg

##### New Member
chin, you don't have 50 feet of wire and a 9 volt battery? I'm disappointed.
9volt battery's short circuit current is like 1A, at least the one I have. 50meters of wire my wires have a resistance of 0.17R per m. 50 meter is 8R, plus you need another 50m to complete the circuit. that's 16R. so the max power I can transfer is 5W. can't even melt steel wool. but anyways I'm getting an old phone. that means infinite range + extreme accuracy.

#### chingyg

##### New Member
takes balls =) Walking up to one after a misffire takes even bigger ones =O =\
It ain't balls, it is sheer teenager's stupidity.

#### Torben

##### Well-Known Member
It ain't balls, it is sheer teenager's stupidity.
hehe. When I was 8 I was experimenting with model rocket engines in my bedroom. Sheer ignorance led me to the point where I accidentally ignited one without securing it to anything. Those things really only burn for a second or two but that was just about the longest two seconds of my life.

Torben

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