LED Pulse Generator using 555

[WindWalker]

Hi,

I'm trying to build a circuit using a 555 as astable multivibrator to generate a square wave of frequency >= 10 kHz and duty cycle HIGH >=90% (duty cycle LOW <= 10%) in order to drive PNP BJT.

I've got so far on MultiSim as this:

The real C1 that I have is 0,9 nF, R3~106.3k so the frequency is theorically ~13.63 kHz (in reality, I think it will be less due to circuit losses/non idealities) and theorical maximum duty cycle LOW ~9.46%. In the schematics I'm using a 1uF capacitor because I can't synchronize the signals on the oscilloscopes (I started to use MultiSim less than a week ago).

Also, the PNP transistors I'm supposed to use are S9012 (Ic_max = 500 mA), LED pulses are generated to get a maximum 100 mA current value in a two LED series. I'm using PNP because all the complementary NPN I have (S9013) have lower hFE measurements (~300 vs ~220), still I can use either.

LEDs will be white so ~4.5V at 100mA are expected (very uncertain about this, it's just data collected from the web). I have very little clue about calculating R4 value. I read the PNP transistor should function in satured mode in order to dissipate very little power (Vec_Sat ~ 0.2V times 0.5A results in 100mW).

R5 value should be (Vsupply-2*VLED-VECsat)/.1=(12-2*4.5-0.2)/.1=28 Ohm, right?


Thank you.

 

[BrownOut]

Replace D1 and D2 with a piece of wire. Also, there should be no resistance between C1 and pin 7.

 

[WindWalker]

Thinking about how the 555 works in astable mode, I can see why diodes D1 and D2 are useless.

If I remove R1 resistor, will it not result in too much current when C1 discharges and/or duty cycle uncertainty when C1 'sees' a short circuit to pin 7 (because internal equivalent resistance to ground is not zero)?


Thanks

 

[BrownOut]

My bad. What I meant to say was the pins 6,2 should connect to pin 7 directly. But keep the resistance between C1 and pin 7.

 

[MikeMl]

Your circuit has another fundamental flaw. What limits the peak current through the LEDs when the PNP turns on????? (Hint: 10mΩ ain't enough)

 

[WindWalker]

Hmm... that way the 'seen resistance' when C1 discharges is (Rpot+R1) in parallel with a short circuit, which still results in 'seeing' a short circuit and only the IC internal resistance limiting the current :S

Your circuit has another fundamental flaw. What limits the peak current through the LEDs when the PNP turns on????? (Hint: 10mΩ ain't enough)

R5 is not supposed to be 10mΩ. I've calculated its value in the first post: "R5 value should be (Vsupply-2*VLED-VEBsat)/.1=(12-2*4.5-0.7)/.1=23 Ohm, right?"

 

[BrownOut]

No! Disconnect pins 6,2 from C1 and connect them to the pin 7 node instead.

 

[WindWalker]

I got this now:

I guess the threshold voltages are changed, so duty cycle and frequency will be slightly different, though I think the difference is minimal.

Should using a 900pF ceramic capacitor as C1 be a problem in any way?


Thank you

 

[BrownOut]

Windwalker, I have you some really bad advise. After reviewing your circuit and looking back over typical 555 circuits, what I should have said was to eliminate R1 (from the original circuit) and connect pin 7 of your timer to the junction of the two timing resistors, R3 and R2. The reason for this is that node should be 0V during discharge to insure the capacitor can drive towards 0V and thus low threshold ( Vcc/3) will always be gaurenteed. Sorry if I confused you. You may PM me if you want a better explaination. Also, you can add as many caps to C1 as you like.

 

[WindWalker]

I'm not worried, I already have a spare 555 and a new one (low power CMOS) costs 0.42€ (0.30$)

If I did understand you correctly, the new schematics should look like this:

Although I'm an Electrical Engineering student, i didn't understand your explanation quite well; by 'that node' you mean the node involving pin 7 of the schematics just above? Don't worry about giving details, I study them all the time

By 'you can add as many caps to C1 as you like' you mean that I can put several capacitors in parallel to get an equivalent capacitance that is the sum of individual capacitances? That I already knew hehe.

 

[Chippie]

R5 is still too small to limit the current through the leds.... 33 ohms..Assuming the transistor is hard on and no volt drop across the CE junctions

 

[WindWalker]

I knew that, but... done.


BTW, I found a 'saturation calculator'.

For the Rc resistor, like Chippie said, assuming no voltage drop across E-B-C (Vce_sat=0) we get Rc=(Vsupply-2*VLED)/Ic=(12-2*4.5)/.1=30Ω.

Considering minimum gain of 50, Ib should be at least Ic/minimum_gain=.1/55 and Rb<(Vsupply-VEBminimum)/Ib=(12-0.6)/(.1/55)=6.27kΩ => ~6.2kΩ?


If I got 5 two LED series in parallel (Ic=500mA, minimum gain of 40), Rc would have the same value and Rb<912Ω => ~820Ω?

 

[BrownOut]

Hi WindWalker,

Here is the drawing that I could not attach to the PM. Also, here is a great turtorial. **broken link removed**

 

[WindWalker]

The people who are in charge of the Electronics Laboratory of my University let me use an oscilloscope

The schematics is the second I posted here, with capacitor C2 absent.

R1 - 974Ω (nominal 1kΩ)
R2 - 10.19kΩ (nominal 10kΩ)
R3 - 11.98kΩ + 104.4kΩ (nominal 12kΩ + nominal 100kΩ)

C1 - nominal 1nF

Power Supply is some SMPS taken from damaged external hard disk, +12V 2A +5V 2A

+12V rail - no load

+12V rail - incandescent bulb (~40mA I think) + 2 series LED @ ~8.50mA (pulsed mode) + 3 series LED @ 8.50mA (continuous mode)

555 Output Pin - no load

Don't like those spikes at all...

555 Output Pin - sinking around 2mA when output is LOW

10% duty cycle LOW

Capacitor C1 voltage:

Frequency ~6kHz

LED current limiting resistor voltage (nominal 47Ω):

More detailed:

PNP S9012 base resistor voltage (nominal 5.6kΩ):

Transistor Emitter-Collector voltage:

Transistor Base-Collector voltage:

LED1 voltage:

LED2 voltage:

Resulting frequency should be above 10kHz. I think the cause is the parasitic inductances due to wire/lead length (the capacitor C1 still has its original long leads) or the oscilloscpe input impedance (input capacitance).

Anyway, I'm quitting this pulsed mode LED idea and going to much simpler PWM control. Why? Well:

Human vision is nonlinear, but that nonlinearity is after a surprisingly accurate time-integration process. When a light is flashing rapidly enough to appear continuously on without flicker, what you see has a good correlation (although nonlinear) with average brightness and is surprisingly independent of peak brightness.

Pulsing LEDs to make them look brighter?

The average user of LEDs cares only about how bright the light is. The eye is slow to react, and registers the average brightness of the light. Although it is true that we have increased the instantaneous brightness of the light, average brightness (that the eye sees) is the same.

Powering Light Emitting Diodes (LEDs)

And I verified this myself by biasing a 3 LED series with a resistor to get the same average current value (~8.50 mA) of the pulse driven LEDs.