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Multiple State LED Flasher

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klbullard

New Member
I am looking for advice on the best/most efficient way to set up a sequence of led flashing events without the use of a microprocessor. The start of the sequence will be initiated by an optical sensor detecting a switch on position. The sensor output will remain high in the on position and low in the off position.

Once triggered the single led will turn on steady for 1 minute and then begin flashing at 1 flash per second for 1 minute (on 1 sec/off 1 sec). At the end of that minute it will start flashing at a rate of 1 flash per 250ms (on 250ms/off 250ms) and remain in this state until the switch is moved to the off position. So the led conditions are:

  • Off
  • Steady On
  • 1 Sec Flash
  • ¼ Sec Flash
  • Off

I am thinking I can do this with a couple of 556 timers but am not sure how to trigger the sequences. Any advice would be greatly appreciated or perhaps 555's are not the best route to take. Again I would appreciate any advice. Thanks to all.
 
Last edited:

Sceadwian

Banned
Why are you avoiding using a micro controller? The circuit complexity drops by an order of magnitude if you do.
 

dougy83

Well-Known Member
Why are you avoiding using a micro controller? The circuit complexity drops by an order of magnitude if you do.
Why do it the easy way when you can do it the ... well, the easy way.

See attached. Component values will need tweaking depending on how pedantic you are re timing.
 

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Sceadwian

Banned
dougy, I'm not much of a discrete logic guy, can you put down a few paragraphs about how that circuit functions? I wouldn't think you could get such complex behavior out of such a simple circuit.
 

dougy83

Well-Known Member
dougy, I'm not much of a discrete logic guy, can you put down a few paragraphs about how that circuit functions?
Well... this is probably going to be harder than drawing the cct...

With the switch open, the LED won't operate. Also, c2 & c1 are discharged. So, u1a out is high, u1b out is low, u1c out is high. u1f out is low due to high through d6. Both oscillators are disabled by the high levels through diodes to their inputs.

When the switch closes, the led conducts. after 1 minute, c2 charges enough to flip u1a output to low. So d4 & d6 no longer conduct. u1e is now able to oscillate @ 1Hz, which pulses the LED at this frequency.

After 1 more minute, c1 is charged enough to flip u1b output high. u1e osc is disabled through d5. u1c output goes low, u1d osc is enable as d3 no longer conducts. u1d osc pulses led @ 4Hz. From now on nothing new happens, so 4Hz pulse continues.

When switch is opened again (LED off - C2 may discharge through it), C2 discharges through d1 & r3, then c2 discharges through d2, now everythings back to the original state.


simple circuit.
Simple, what? There's enough diodes to make a small uC!:D
 

Sceadwian

Banned
Amazing what you can do with a Schmitt Inverter and a handful of discrete though.
 

klbullard

New Member
Why are you avoiding using a micro controller? The circuit complexity drops by an order of magnitude if you do.
It has to do with with the fact that if I use a microprocessor there will be software involved and that will require software validation for this project. I would prefer the processor approach but my requirements state no microprocessor. Thanks for the reply.
 

Sceadwian

Banned
Software validation for a variable blinker? That's silly. Then again people that make certifications up tend to be =)
 

klbullard

New Member
Well... this is probably going to be harder than drawing the cct...

With the switch open, the LED won't operate. Also, c2 & c1 are discharged. So, u1a out is high, u1b out is low, u1c out is high. u1f out is low due to high through d6. Both oscillators are disabled by the high levels through diodes to their inputs.

When the switch closes, the led conducts. after 1 minute, c2 charges enough to flip u1a output to low. So d4 & d6 no longer conduct. u1e is now able to oscillate @ 1Hz, which pulses the LED at this frequency.

After 1 more minute, c1 is charged enough to flip u1b output high. u1e osc is disabled through d5. u1c output goes low, u1d osc is enable as d3 no longer conducts. u1d osc pulses led @ 4Hz. From now on nothing new happens, so 4Hz pulse continues.

When switch is opened again (LED off - C2 may discharge through it), C2 discharges through d1 & r3, then c2 discharges through d2, now everythings back to the original state.


Simple, what? There's enough diodes to make a small uC!:D
Dougy thanks for the circuit explanation and Sceadwian, thanks for asking for it. I would have had to build it first and probably still not fully understood the circuit. I'll build it this evening. I am still curious about the 555 approach. Do you guy's think using the timer is out of the question? Would it take more components or just be less practical?
 

blueroomelectronics

Well-Known Member
The 555 would take more. It's a pretty nifty circuit using only a single IC. (Of course a small 6 pin micro would be simpler)

What's it for?
 

klbullard

New Member
The 555 would take more. It's a pretty nifty circuit using only a single IC. (Of course a small 6 pin micro would be simpler)

What's it for?
Thanks for the insight Blueroom. The circuit will be packaged in a small hand held device used for measuring liquid flow. The different led flash rates are to indicate the passage of time at a high flow rate condition. This is a small part of a larger medical device.
 

dougy83

Well-Known Member
I am still curious about the 555 approach. Do you guy's think using the timer is out of the question? Would it take more components or just be less practical?
All the gates in the proposed cct can be in the same 14-pin IC. You could replace each of the schmitt trigger gates with a single 555. You could possibly redesign it to use 3 x 555 (2 for timing & 1 for dual-speed oscillator), but I don't see the point; the 555 approach will be bigger, and consume more power. The fact that it is a 'small hand-held' device would generally mean that both size & dissipation must be limited.

It has to do with with the fact that if I use a microprocessor there will be software involved and that will require software validation for this project. I would prefer the processor approach but my requirements state no microprocessor.
Who sets the requirements? Surely, as the designer/engineer, you can negotiate with the client if you think a certain approach would be better.
 

Sceadwian

Banned
If you're measuring liquid flow that's likely there's already a micro controller involved in this project anyways? Something has to be doing the reading.
 

klbullard

New Member
If you're measuring liquid flow that's likely there's already a micro controller involved in this project anyways? Something has to be doing the reading.
No microprocessor Sceadwian. The device is measuring flow in one of three states, hi, low and no flow and is done with discreet components only. It is a time of flight measurement using a comparator and thermistors. The output condition is displayed through led's. The part of the circuit I am working on is a flashing indicator letting the user know how long the device has been in a high flow condition.
 

klbullard

New Member
You could possibly redesign it to use 3 x 555 (2 for timing & 1 for dual-speed oscillator), but I don't see the point; the 555 approach will be bigger, and consume more power.
This is the approach I was contemplating before posting here. I agree, I dont see the point after seeing your solution.

Who sets the requirements? Surely, as the designer/engineer, you can negotiate with the client if you think a certain approach would be better.
The client has set the requirements and for the section I am working on this translates to no microprocessor and the led's must flash as indicated. Timing is not that critical as it is a visual indicator. Thanks for your help on this I was looking for a more practical solution and this seems to be it. I'll let you know how it breadboards out.
 

tcmtech

Banned
Most Helpful Member
Ah medical devices. Under $3 to build and likely $300 or more to buy!
Capitalistic greed at its finest!
 

klbullard

New Member
Well... this is probably going to be harder than drawing the cct...

With the switch open, the LED won't operate. Also, c2 & c1 are discharged. So, u1a out is high, u1b out is low, u1c out is high. u1f out is low due to high through d6. Both oscillators are disabled by the high levels through diodes to their inputs.

When the switch closes, the led conducts. after 1 minute, c2 charges enough to flip u1a output to low. So d4 & d6 no longer conduct. u1e is now able to oscillate @ 1Hz, which pulses the LED at this frequency.

After 1 more minute, c1 is charged enough to flip u1b output high. u1e osc is disabled through d5. u1c output goes low, u1d osc is enable as d3 no longer conducts. u1d osc pulses led @ 4Hz. From now on nothing new happens, so 4Hz pulse continues.

When switch is opened again (LED off - C2 may discharge through it), C2 discharges through d1 & r3, then c2 discharges through d2, now everythings back to the original state.


Simple, what? There's enough diodes to make a small uC!:D
Dougy does the schmitt trigger need to be of the cmos variety for this circuit?
 

Sceadwian

Banned
Yes it needs to be cmos.
 
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