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555 timer reset

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ZOR100

New Member
I have a beam break detector circuit which drives the circiut belowView attachment 64551

The output of the timer pin 3 drives a relay making an alarm ring. However, I now realise the alarm cannot be on forever, so I need to have a timer to turn it off. I realise now I should have just had a variable timer circuit doing the operation in one go.

However I have now made another timer using a 555, which I intended to start up at the same time as the circuit above is triggered, and use it's output to put a low on the circuit above reset line.

But, I have a worry that if the output of timer two is strapped accross the reset line to begin with, then the first circuit will reset before it's started. Just wondering if anyone has an idea other than me messing my stripboard up even further to convert my first circuit into a straight timer.

Thanks
 

eTech

Well-Known Member
I have a beam break detector circuit which drives the circiut belowView attachment 64551

The output of the timer pin 3 drives a relay making an alarm ring. However, I now realise the alarm cannot be on forever, so I need to have a timer to turn it off. I realise now I should have just had a variable timer circuit doing the operation in one go.

However I have now made another timer using a 555, which I intended to start up at the same time as the circuit above is triggered, and use it's output to put a low on the circuit above reset line.

But, I have a worry that if the output of timer two is strapped accross the reset line to begin with, then the first circuit will reset before it's started. Just wondering if anyone has an idea other than me messing my stripboard up even further to convert my first circuit into a straight timer.
Thanks

if I understand the problem correctly, what you really need is a monostable 555 circuit with reset capability.
You could configure the timer so that it would energize the relay for a predetermined amount of time and reset automatically or you can press the reset button and reset it manually. Is this what you need?
 

eTech

Well-Known Member
I have a beam break detector circuit which drives the circiut belowView attachment 64551

The output of the timer pin 3 drives a relay making an alarm ring. However, I now realise the alarm cannot be on forever, so I need to have a timer to turn it off. I realise now I should have just had a variable timer circuit doing the operation in one go.

However I have now made another timer using a 555, which I intended to start up at the same time as the circuit above is triggered, and use it's output to put a low on the circuit above reset line.

But, I have a worry that if the output of timer two is strapped accross the reset line to begin with, then the first circuit will reset before it's started. Just wondering if anyone has an idea other than me messing my stripboard up even further to convert my first circuit into a straight timer.
Thanks

if I understand the problem correctly, what you really need is a monostable 555 circuit with reset capability.
You could configure the timer so that it would energize the relay for a predetermined amount of time and reset automatically or you can press the reset button and reset it manually. Is this what you need?
 

alec_t

Well-Known Member
Most Helpful Member
You don't need a second 555.
Instead of grounding pin 6 connect it to the + terminal of a cap (say22uF). Ground the - terminal of the cap. Connect a resistor (say 470k) from +V to the cap +. Connect the cap + via ~10k to pin 7. That should give you an alarm sounding for ~10 secs. Adjust the cap or 470k values for other timings.
 

ZOR100

New Member
Thanks eTech, yes you are correct, I am trying to modify the circuit attached as little as needed to avoid my stripboard really coming to pieces second time round. I am wanted to have the reset done automatically by set time.

Thanks alec_t, I m just looking at what you say, little bit lost on the latter part ie 10k etc. I will try and read it slowly again offline.

Regards
 

eTech

Well-Known Member
Thanks eTech, yes you are correct, I am trying to modify the circuit attached as little as needed to avoid my stripboard really coming to pieces second time round. I am wanted to have the reset done automatically by set time.

Thanks alec_t, I m just looking at what you say, little bit lost on the latter part ie 10k etc. I will try and read it slowly again offline.

Regards

Hi ZOR..

Ok...then alec_t's correct, you don't need another 555 circuit.
You should be able to modify your existing 555 circuit to get the functionality you need.

eT
 

ZOR100

New Member
Many thanks both, I understand now, pin 6 to go to a capacitor to ground, and pin 6 to goto a resistor to +ve. I will play with resitor/capactitor values, and will put last 555 back in my goody box. Hope that should get me sorted, have a good weekend both.
 

ZOR100

New Member
Thats it, managed to modify my stripboard without fatality and it all works a treat. In the end because of space, and fortunately works, I left 6 and 7 joined so thanks again.

Must investigate why my pc keeps leaving out letters when I am typing, it's not the keyboard because others do it as well. Must find another forum to see whats what.
 

harold777

New Member
ZOR100 I have noticed this today, but being a "newbie" though I'd done something wrong.

Only noticed it in EDIT-mode (ie after 1st posting), so maybe there is abug flying about?
For me it was taking several words and pasting them a little further along . . . odd that!

On the 555 - be careful with lare electrolytics, they can mess up lengthy time intervals when they start to leak. I'd favour a digital solution using 4017, it uses more parts, but no large C's. A 555 can supply the clock.
 

ZOR100

New Member
Thanks Harold, yes I noticed large capacitors start leaking before charging so it never gets there. I used a trimmer to the +volt rail to trim time, but included a fixed resistor to stop the 555 getting full rail voltage, doesn't like that!!

Re my keyboard missing letters, seen lots of it on google but have not seen a real fix. Even hammering the keyboard makes no difference. Never mind. Thanks again
 

harold777

New Member
Provided the supply isn't above the 555 spec... it's not good practice to boost the voltage to the capacitor if you are relying on the leakage to hold the voltage lower than the 555 supply.
There is a eal chance that sooner of later the C will stop leaking and the voltage blows the 555, but obviously this depends where/how the componnts are connected. To connect a 555 to a supply that's is too high your only solution is to fit a resistor and a zener across the 555...... but it not a "nice" solution.

What did you think of the 4017 counter?
An alternative cct uses the 4060 which provides both clock-osc and counter, but you'll see there are some divisions missing . . . but it's handy if you just need a slow oscillator unig film capacitors . . . . electrolytics are always bad news IMHO. and Tants can become destroyed by damp entering by the epoxy round the legs, this is where they are often damaged while soldering-in. 4060's can supply several outputs but they are divisions by 2 . . . but it's compact.
 
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