Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

another 555 question

Status
Not open for further replies.

daviddoria

New Member
say i wanted a monostable circuit (a good example i saw was a rocket launcher), but instead of when i pushed the button it to go high, i wanted it to go low for a certain time so i could run away from the rocket and then it would launch automatically.

how would i do this?
 
interesting....

but in the case of a rocket.... before i pushed the button for it to go low so i could run away... it would already be high... and i would get a rocket in the face. i need something that is low.... i push the button... it stays low... then goes high after a time interval.

thanks
david
 
daviddoria said:
interesting....

but in the case of a rocket.... before i pushed the button for it to go low so i could run away... it would already be high... and i would get a rocket in the face. i need something that is low.... i push the button... it stays low... then goes high after a time interval.

thanks
david

how bout a when you press the timer start and the 555 goes high, it triggers the second circuit to start ready for when it goes low, then a NOT gate turns to high
 
You can use a 556 the pinouts are different. There are two other issues
1 The 556 ( or 555) disn't have enough current to heat the nichrome wire ( I assume thats what Your using).
2 The output pulse isnt long enough to fire the wire either.
Both can be overcome by using a power transistor with a resistor and cap from base to ground. The cap will charge when it recieves the pulse from and will discharge thru the resistor 1uf and 20k is good isolate the 556 output from the transistor with a 4001 diode ( so the cap dosn't discharge thru the 556 ). The Transistor can be a 2N3055 or equilv, it's overkill but hey so what. IIRC the nicrhome is fired by 4 d cells? Use a NE 556 and the supply V can be higher (3-15V)
 

Attachments

  • LM555Delays2.gif
    LM555Delays2.gif
    9.1 KB · Views: 543
i'm sorry i dont follow what you're saying.
what do you mean by "triggers the second circuit to start ready for when it goes low"?
 
hmm in that circuit... doesn't chip A just do the same to chip B as the switch did to chip A?
 
daviddoria said:
i'm sorry i dont follow what you're saying.
what do you mean by "triggers the second circuit to start ready for when it goes low"?
ignore me... i normally do things the hard way... try TillEulenspiegel's idea instead...
 
daviddoria said:
hmm in that circuit... doesn't chip A just do the same to chip B as the switch did to chip A?
Yes ;)
One thing I didnt mention You can put a "safty" switch between the collector of the transistor and the supply
 
i dont think a cap would be necessary, or the protector diode. i can just run the output to the base of a NPN transistor, +v to the collector, and emmiter to the lighting wire.

however, i still dont understand how you get some low time before some high time...
 
The cap/resistor circuit at stated values will give you a 2 sec pulse. The diode is necessary because after the positive pulse the line returns to ground and if the base ( before the R/C components) is not isolated the cap will discharge thrui the 2nd half of the 556. The r/c is just a "pulse stretcher" i.e. keeps the transistor on longer then the short pulse.

The chip is edge triggered by a negative going pulse
 
what does "edge triggered" mean?

also, i will just change the values on the 2nd 555 to make the pulse long enough to ignite the thing, what i need to understand is what the 2 chips does better for me than just 1
 
Edge triggered means that a chip is actuated by a transition from one state to another. Look a trigger pulse one in the timing diagram, now at TP2. See the pulse go from + to - ? thats it neg edge ( or falling ) , some chips have positive ( or rising edge ) edge triggering

The reason 2 is better then one is that as you stated one will fire immeaditly...so we use the first 1/2 of the 556 to be a timer it times and fires a pulse to the 2nd half of the 556 and keeps it on by the r/c constant .
 
ok i'm getting close... i think those waveforms are exactly what i want. however, i dont understand still....ok at the very beginning both outputs are low

when you push the switch, pin 2a is edge triggered in to making pin 3a high... but upon doing this doesn't pin 2b get edge triggered into making pin 3b high?? it seems to me like both outputs should always be at the same state?
 
but upon doing this doesn't pin 2b get edge triggered into making pin 3b high?? it seems to me like both outputs should always be at the same state?
No remember it is negative edge triggered! The time delay in 556a goes high until the charge in it's circuit discharges when that happens it fires making a neg pulse that triggers 556b. The first timer is a "push and stay on " until its r/c circuit discharges...it won't affect 556b until the input of that chip is triggered by a negative going pulse.
You may want to look up the theory on the 555 . It's simple, just a couple of comparators a resistive divider, an r/c ( outboard ), a flip f;op and an output
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top