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| floobydust:
The flasher circuit is also on Wikipedia https://en.wikipedia.org/wiki/Forrest_Mims#Author |
| james_s:
I remember seeing a very similar flasher circuit in those warning lights they used to put on construction barricades. The old ones has an incandescent lamp and used a pair of 6V lantern batteries in parallel. |
| Jwillis:
--- Quote from: fourfathom on August 16, 2019, 12:43:00 am --- --- Quote from: Jwillis on August 15, 2019, 10:40:56 pm ---I tried your flasher circuit and all it did was cook the transistors. So I took the liberty of redesigning the circuit to one that I tested and indeed works and it won't cook the transistor.I added notes but feel free to make any changes you like for different effects.I was to lazy to diagnose the problem with the first circuit. --- End quote --- This circuit has a problem too, in that when the NPN turns on there is no limit on the current from 9V+, through the LED, through the PNP emitter-base junction, through the NPN collector-emitter path, to ground. Perhaps there isn't enough base current into the NPN to drive things hard enough to cause problems, but I would like to seen some current limiting, probably at the PNP base. --- End quote --- The current to the LED is limited by the 330-470 ohm resistor.Because the LED has a forward voltage of 1.7V the current it draws is I= (9-1.7)/330= 22ma ,Although that seems high the actual is lower because the battery is not at full potential (8.5V) . That puts the current at around 20mA . Since the LED is on a pulse ,short bursts of slightly higher current isn't hurting the LED .This has been measured and confirmed .The 470ohm resistor would be better limiting the LED to around 15.5mA .This is also the reason that no limiting resistor is require at the base of the PNP transistor.Because only 20mA is being drawn across the transistor the current at the base is very small.About 0.5mA has been measured at most. |
| floobydust:
In the circuit https://www.eevblog.com/forum/beginners/transistors-204336/msg2618643/#msg2618643, I think the off-delay potentiometer needs a current-limiting resistor and I still see a high-current path through the LED from base current (E-B junction) for Q2. If Q1 ever stays on, the LED is roasted. This construction barrier flashers I've seen were the incandescent light bulb with a bi-metallic blinky switch inside and a 6V lantern battery. Newer ones are LED and not as bright. |
| Jwillis:
--- Quote from: floobydust on August 17, 2019, 04:06:24 am --- In the circuit https://www.eevblog.com/forum/beginners/transistors-204336/msg2618643/#msg2618643, I think the off-delay potentiometer needs a current-limiting resistor and I still see a high-current path through the LED from base current (E-B junction) for Q2. If Q1 ever stays on, the LED is roasted. This construction barrier flashers I've seen were the incandescent light bulb with a bi-metallic blinky switch inside and a 6V lantern battery. Newer ones are LED and not as bright. --- End quote --- It was a quick thrown together circuit but your right if the resistance gets to low on that pot it would cause the LED to fail .But as I mentioned any value less than 3Meg theirs no visually noticeable delay. So it may be better to have a 3Meg resistor and a 5Meg pot .If a faster flash is required then lowering the value of the cap would do that. I had it oscillating with a 470pF chiclet. Not noticeable to the eye but just wanted to see what was going happening on the scope. Few extra resistors here and there probably won't hurt performance . |
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