Hi,

Schmitt triggers have hysteresis, that's why the unequal on/off times.

Actually, it's not the hysteresis, but a thing happening when the threshold voltages aren't symmetrical around Vdd/2 (0V reference assumed).

I think there is a way to get an ~50% duty cycle with the one Schmitt NAND gate oscillator. I think you have two sets of feedback resistors/pots. A diode is placed in series with each set. the diodes point in opposite directions.

Just adjust the load until you satisfy the above

The surefire way of getting 1:1 is of course using double the frequency and then divide it by 2 in a flip-flop.

 

I think you have two sets of feedback resistors/pots.

Hmm.. no, I test for each frequency by varying the preset, if it seems good, replaces it with a resistor. I use no more than 1 preset at the same time.

To get a M/S ratio of 1:1, perhaps try another 4093 (preferably from another manufacturer), or adjust the load on its output to get V_tH and V_tL symmetrical around Vdd/2.

Thats right, cos the first time when I powered it up, I used LED's from the TIP's and found that one LED would only dim, not go off at all. Then I loaded them with bulbs and everything became normal.

The surefire way of getting 1:1 is of course using double the frequency and then divide it by 2 in a flip-flop.

Duty cycle is not of much importance in this app as the more weird it blinks, the more outstanding the circuit becomes. If it was some precision application we would have used precision timers. So it isnt really a problem right?

 

I think you have two sets of feedback resistors/pots.

Hmm.. no, I test for each frequency by varying the preset, if it seems good, replaces it with a resistor. I use no more than 1 preset at the same time.

This is to set the duty cycle. These are paralleled, replacing the single "preset" timing resistor.
---/\/\/\--|<---

---/\/\/\-->|---
Because of the diodes' orientation, the upper resistor sets the charge time and the lower resistor sets the discharge time. So you can tweak your duty cycle. They will slightly affect the frequency, as you change one resistor value or the other.

Ken

 

Alright, let me see..

 

I did a quick work and made a small board...

 

Pretty and compact!...but does it work?

Ken

 

Ya it works fine and is cool cos of the heatsink. Without it, the TIP wont survive the 4A load. Thanks to you guys. I need to get a box to mount this thing in. I have limited it to 3 flashes to minimize the stunning effect.

 

Congratulations on a successful project.

Ken

 

It seems I have a small doubt to clarify. I tested the circuit with 2 10W bulbs and now before fixing it permenantly into my car, I find that the darlington is getting very hot with the heat sink on my car's circuit. One heat sink a bit more hotter. One bulb pair draws a bit over 3A and the other around ~4.15A totaling about 7.5A. I used the TIP127 taking into account that it can handle 60W and 5A. With the current load and the heating, will I need to replace the TIP with higher ones ? And also is the heatsink in the image ok to handle a hot TO-220 chip or will I need a bigger one? i needed to know it before I fix it permanently. Hot heatsink is not an issue for me as long as it doesn't burn out.

One other doubt I have is that the wire I used to tap the brake line temporarily, was not a good one. When I connect the 2 bulb pairs (7.5A) directly together through this wire, it heats up lightly, but when I used the circuit, it melted the plastic coating of the wire after about 30 seconds. Why didn't the same happen when i connect them directly? I have double checked for short circuit and the circuit carries a 10A fuse, so that wouldn't be an issue. Could the wire gauge used in the circuit be the problem also for heating the TIP?

 

Well, I replaced Vcc and output wires with 1sq.mm industrial grade wire and now all most everything is solved except my doubt about the heatsink. One of the TIP heats up like soldering iron and even smells like it!, should I go for a TIP 137/147 or a larger heatsink? Its ok even with the current one, but I doubt if it would burn the TIP. If I swap the outputs, the other TIP heats too much. One of my load is obviousely heavier. I ran it for 1min continuous, nothing happened other than getting too hot, would it be a problem? One another possibility is that I ran the test with the engine off, so voltage at around over 11V.

P = VI

V(normal) x I(normal) = V(low) x I(high?)

Could that be a reason that since the supply voltage is low, the bulbs are drawing more current than it should and would become ok when the engine runs. I would only be using it on a running engine.

Other than all this the circuit works good with no problem.

 

JJ,
Can you post a schematic, with values, of the circuit you're currently using.
Ken

 

Same last circuit you posted. Components changed are:-
1. Transistors BC547
2. Darlington TIP127 for more current handling
3. Diodes 1N4007 It was what I had in hand.
4. And a resistor in the second oscillator from 10k to 100k to adjust frequency.
Load first output around 3.2A and second around 4.2A
Tested only with engine off, little over 11v maybe (didnt check)
Circuit is good and functional with light loads 21+10W bulbs tested at 13V with mild heating only on the 21W load (1.5A) from my test battery.
When tested on car 4.2A loaded TIP heats verymuch, 3.2A loaded one heats less.
With wires reversed, the pattern reverses.
Tested on car for abt 1 min with both loads, heating problem only, no burning.
I think it calls for a single TIP replacement or a bigger heatsink.

What would you suggest for this Ken?

 

Which transistor, Q2 or Q4, is generating the most heat?

ken

 

I "think" I see the problem.
http://www.fairchildsemi.com/ds/TI%2FTIP127.pdf
The datasheet for the TIP127 shows Vce(SAT) at 2V@Ic=3A with 12mA base current and 4V@Ic=5A with 20mA base current. The base resistors R9 and R11 are only supplying about 4mA to turn on the transistors. The transistors are not being fully turned on, so a lot of power is turned into heat. Try changing R9 and R11 to 560Ω 1/2W.

Ken

 

Oh man, you are great. I didnt even think about it. 0.5W's are not in stock, I'll get them in an hour and try again.

Q2 heats up too much if the 'heavier' load is connected to it and at that time, Q4 heats like a normal power transistor. If the loads are inversed, the pattern reverses.

 

Ken, thanks a ton.. The TIP's now heat as it should as far as I know from my experience using TO-220 case devices. Also the biggest noticeable change was that with the 2k7, the bulb wouldn't light up similar to my test bulbs and the flashing was a bit dull, so I though it was due to the thermal latency of the bulb or overload. But now with the 560R, I understand why it happened. It flashes clean and merges as a single one as if nothing has happened. Anyway, a project ended well. I moulded out a PVC case and installed them onto my car. Anyway here is the final circuit. Thanks again Ken, you are simply great!

 

Thanks for the opportunity to play!

Ken

 

Hihi.. You're welcome..

 

The flasher circuit looks real good for light bulbs. What modifications need to be made to do the same thing using LEDs? There isn't nearly the current requirement and I doubt that a heat sink will be necessary.

 

Basically nothing, if you're using 12V automotive LED lamps. If you want to just use LEDs you have to add an appropriate dropping resistor in series with the LED to replace the light bulbs.

ken