Need help with using transistors as a switch with 555 timer

[amzjl]

If I'm basically using the transistor like a zener, why can't I just use a zener?

 

[Audioguru]

Use a low current zener diode instead of the two-wires transistor.

 

[amzjl]

What counts as a low voltage? I found some ranging from 3 to 12 volts. Does my selection depend on the voltage I am using? What I noticed from the product descriptions the higher the voltage, the lower impedance (ohms) value. What does that mean?

 

[Audioguru]

I said to use a low current zener diode but you are talking about a low voltage one. Current and voltage are completely different.
The impedance of a zener diode is lowest when the current is high and the zener voltage is about 6V.

The transistors remain turned off while the capacitor is charging. It takes time for it to charge. When the capacitor voltage reaches the zener voltage plus the base-emitter voltages of the transistors then the transistors turn on.

 

[amzjl]

That's why I brought it up. I noticed looking at the parts the higher voltage models had lower impedance. I didn't understand the reverse correlation of voltage to current. I know they are different. I don't want to buy the wrong thing and didn't how low 'too low' was. I'm assuming the lower the impedance, the shorter the time interval.

 

[Audioguru]

The time delay is caused by the charging time of the capacitor for its voltage to reach the zener voltage plus the small base-emitter voltages of the transistors. Therfore the time interval is determined by the voltage rating of the zener diode, not its impedance.

You are WRONG about the impedance rating of zener diodes at different voltage ratings. The impedance is the lowest at about 6V and is higher at lower and at higher voltages. The impedance is also lowest when the current is higher in relation to the resistor value that powers it.

Here are the datasheet spec's of two ranges of zener diodes:
1) The BZX79cxx are rated at only 5mA.
The max impedance at 2.4V is 100 ohms, at 6.2V is 10 ohms and at 56V is 200 ohms.

2) The 1N52xx are rated at 20mA for most but the current is reduced at higher voltages so that they do not overheat.
The max impedance at 2.4V is 30 ohms, at 6.8V is 5 ohms and at 56V is 150 ohms.

 

[amzjl]

So how do I pick the correct zener? I can figure out the time portion but I am assuming I need to know the zener first.

 

[Audioguru]

The two transistors have a total voltage loss of about 1.8V. If your supply is 6V, your LEDs are 3.4V white ones then the voltage across the current-limiting resistor for 20mA is (6V - 1.8V - 3.4V)/20mA= 0.8V which is not enough voltage for the resistor. With a 9V supply then the current-limiting resistor for 20mA is (9V - 1.8V - 3.4V)/20mA= 190 ohms. Use 180 ohms or 200 ohms.

The transistors begin to turn on when their base voltage of the first one is about +1.3V so if the capacitor is allowed to charge to 63% of 9V (5.7V) then the zener diode should be 5.7V - 1.3V= 4.4V. Use a 4.3V zener diode.

If the load on the transistors is 120mA then the base current of the first transistor is about 0.3mA so the resistor charging the capacitor should have a value that is (9V - 5.7V)/0.3mA= 11k. use 12k. Then use a capacitor with a high enough value for your delay time. 12k x C in Farads= seconds.

It is a crude and simple design so the LEDs will slowly brighten up to full brightness and there is almost nothing to discharge the capacitor after it is not used anymore.

 

[amzjl]

How slow is slow? I need them on immediately after the delay at full brightness. Does this type of circuit require the use of the darlington transistors? Can it just use a zener and one transistor?

 

[Audioguru]

The extremely simple circuit does not have an electronic switch, instead it has a zener diode and two transistors that are turned on by the slowly rising voltage on the capacitor. Try it to see how slow the brightness is. A 555 timer IC has a fast switching comparator circuit.

Two transistors connected like that make a darlington transistor.

If you use a single transistor then the capacitor value must be about 30 times higher and the resistor that feeds it must have about 30 times less resistance.

 

[amzjl]

Can you walk me through how to use the 555 to get what I want? I originally started with it so I understand certain things. I just don't know what hardware to use to make it delay on.

 

[Audioguru]

A 555 timer starts when it is triggered. Then it turns something on for the time period.
I think you want it to keep something turned off when the 555 is powered then the something is turned on at the end of the time period.

I haven't tried it but if the trigger pin 2 has a discharged capacitor to ground then it might trigger the 555 when the power is applied and it begins timing with its output high. You can use the high to turn something off. At the end of the time period the output goes low which can turn the something off.

The capacitor on pin 2 must have a resistor to the positive supply to charge this capacitor. Something must discharge this capacitor somehow when the power is turned off.

 

[amzjl]

I will try to get a better picture of it but in my first post I have that same type of circuit drawn. The output was going to two transistors, a npn connected to a pnp. Output goes high and allows power to flow through the npn to turn off power to the pnp. Timing interval ends, npn goes off, pnp lets power flow through it. The capacitor could be connected to the LED's to ground. Would that discharge the capacitor? My original problem was the transistors had resistors running to them and I didn't know what they were for.

 

[Audioguru]

I will try to get a better picture of it but in my first post I have that same type of circuit drawn. The output was going to two transistors, a npn connected to a pnp. Output goes high and allows power to flow through the npn to turn off power to the pnp. Timing interval ends, npn goes off, pnp lets power flow through it. The capacitor could be connected to the LED's to ground. Would that discharge the capacitor? My original problem was the transistors had resistors running to them and I didn't know what they were for.

I repeat: The NPN in the original circuit prevents the PNP from turning off because the emitter voltage of the NPN does not go high enough. Without the NPN then the Cmos 555 can turn off the PNP. I showed this in my first post.

 

[amzjl]

I know the 555 can turn on the PNP, but it can't do it without the lights coming on first before the timing interval (or so you said). That defeats the entire point of this circuit. If the voltage isn't high enough for the NPN, why can't I just feed it more voltage? how much does it take?

 

[Audioguru]

Why do you have the NPN transistor? It does nothing except prevent the PNP from turning off. The Cmos 555 by itself can turn on and turn off the PNP.
I showed the circuit without the NPN in post #2 and it might do what you want so simply try it.

 

[amzjl]

By your own admittance, the LED's would come on before the timer starts. Is there a way to stop it from happening?

 

[Audioguru]

Your 555 has a capacitor to ground at its trigger pin #2. So when it is powered by the switch the output of the 555 is low then this trigger causes it to go high. I do not know the duration the output is low which might cause the LEDs to make a very short blink. Try it and see.

If the blink is obvious then add a capacitor between the base and emitter of the PNP transistor to slow down its turn on. But this new capacitor will also slow down its turn off.

 

[amzjl]

I was just thinking about using a capacitor for that. If I did, wouldn't it be from collector to emitter or do I have the two types of transistors?

 

[Audioguru]

If you connect a capacitor between the collector and emitter of the PNP transistor then the capacitor will cause the LED to blink as the capacitor charges then the transistor will conduct a high current to discharge the capacitor quickly.
You want the transistor to filter the LED blink so a capacitor from the base to ground will be charged slowly by the series base resistor and prevent the LED blink.
The filter capacitor might work better connected between the collector and the base.