Need help with using transistors as a switch with 555 timer

[amzjl]

So I try it both ways and see what happens. Is there a particular value I should shoot for as far as capacitance? Do I need a pull-up resistor for the PNP?

 

[Audioguru]

Without a capacitor, if the LEDs blink when the power to the 555 is applied then connect a capacitor between the base and emitter or between the collector and base of the PNP transistor as a filter. Try 0.01uF, 0.1uF, 1uF and 10uF and use the lowest value that prevents the LEDs from a blink.

The PNP transistor is turned on through its series base resistor by the output of the Cmos 555 going to 0V and it is turned off by the output of the Cmos 555 going to the supply voltage. Then a pullup resistor is not needed.

 

[amzjl]

If I am connecting a capacitor from either the collector or emitter to base, do I need to worry about any kind of reverse voltage or current going into the chip via the output pin?

 

[Audioguru]

If I am connecting a capacitor from either the collector or emitter to base, do I need to worry about any kind of reverse voltage or current going into the chip via the output pin?

No because there is a series current-limiting resistor from the output of the 555 to the base of the PNP transistor.

 

[amzjl]

I couldn't remember if you or someone else mentioned it before but how do I determine the value of that resistor?

 

[Audioguru]

I couldn't remember if you or someone else mentioned it before but how do I determine the value of that resistor?

The datasheet for almost every little transistor shows that it saturates (turns on like a switch) best when its base current is 1/10th its collector current.
You have six LEDs at 20mA each so their total is 120mA and the base current should be 12mA.

The datasheet for a Cmos 555 shows that with a 12mA load and a 6V supply, its output low voltage is about 0.5V and the base-emitter voltage of the PNP transistor (I looked at the datasheet of a 2N4403 ) is about 0.9V when it is saturated with a 120mA load. So the series base resistor has 12mA and a voltage of (6V - 0.5V - 0.9V=) 4.6v. Ohm's Law calculates the resistor value to be 4.6V/12mA= 383 ohms which is not a standard value so use 390 ohms.

 

[amzjl]

Does the capacitor that is wired from the collector or emitter side to base need to come before or after the resistor on the base?

On a slightly separate issue, do you know how much voltage a typical 555 can take? I was thinking about running more of my LEDs in series so I wouldn't take up a lot of space for batteries, but it would mean a lot of voltage, about 17.5.

 

[Audioguru]

Does the capacitor that is wired from the collector or emitter side to base need to come before or after the resistor on the base?

If the capacitor is connected to the output of the 555 then it will short circuit its output every time it switches then it is not a filter.
instead, connect the capacitor directly from the collector to the emitter or from the base to 0V.

On a slightly separate issue, do you know how much voltage a typical 555 can take? I was thinking about running more of my LEDs in series so I wouldn't take up a lot of space for batteries, but it would mean a lot of voltage, about 17.5.

You DO NOT HAVE a 555. Instead you have a Cmos TLC555 which is very different.

The maximum supply for an ordinary 555 is 18V but they recommend a max of 16V on its datasheet. Its output high does not go high enough to turn of a PNP transistor.
The maximum supply for a TLC555 is also 18V but they recommend a max of 15V.
You should not design a circuit without looking at the datasheets for the parts.

EDIT: DO NOT connect the capacitor from the collector to the emitter to make a filter. Instead, connect the capacitor from the collector to the base. Or connect the capacitor from the base to 0V.

 

[amzjl]

OK, after reading your edit, if I connect the capacitor between the collector and base, does it matter if its before or after the resistor connected to the transistor.

What is the difference the difference between the CMOS and an ordinary 555? How would I go about selecting the correct chip to do what I want?

 

[Audioguru]

OK, after reading your edit, if I connect the capacitor between the collector and base, does it matter if its before or after the resistor connected to the transistor?

The base resistor connects between the output of the 555 and the base of the PNP transistor. You do not want to short circuit the output of the 555 to the collector of the transistor each time it switches, instead you want a filter so connect the capacitor directly to the collector and base of the transistor.

What is the difference the difference between the CMOS and an ordinary 555? How would I go about selecting the correct chip to do what I want?

The datasheets show the differences:
1) The ordinary 555 has an idle current of 6mA to 15mA when it is doing nothing. A Cmos 555 has no idle current.
2) The maximum output current of an ordinary 555 is 200mA source or sink with an output voltage loss of 2.5V. The maximum output current of a Cmos 555 is 0.5mA to 2mA source and is 3mA to 22mA sink depending on the supply voltage ands with an output voltage loss of 2.5V.
3) With no load the output high of an ordinary 555 is 1.3V less than the supply voltage (so it cannot turn off a PNP transistor). With no load the output of a Cmos 555 is rail-to-rail (0V to the supply voltage).
4) The output of an ordinary 555 causes a 400mA supply current spike each time the output switches. The Cmos 555 does not do this.

 

[amzjl]

Is there any resources you know of that can go into more detail about the components I am using and how they operate? I have two books I've been using to walk myself through how to design the circuit. One is very simplistic and the other is way over my head. I have had trouble pulling the info I need to know off the data sheets because I don't quite know what I'm looking at yet. Is there anything I can do so that when I buy the parts, I don't just turn them to smoke.

 

[Audioguru]

I learned about electronics a long time ago from electronic magazines not available today and from university.
If you don't go to university then look at the book, "The Art Of Electronics" that I downloaded from somewhere.

 

[amzjl]

If the voltage I need for the lights is too high for the CMOS, could I create a voltage divider somewhere in the circuit to limit it?

 

[Audioguru]

If the voltage I need for the lights is too high for the CMOS, could I create a voltage divider somewhere in the circuit to limit it?

No, a voltage divider will not work because the base of the PNP transistor must be close to the positive supply voltage for it to turn off.
Instead, the output of the Cmos 555 can drive an NPN transistor through a current-limiting series base resistor. The collector of the NPN can turn on the base of the PNP though a current-limiting series base resistor and a resistor from the base of the PNP to the positive supply (its emitter) can turn it off.

 

[amzjl]

Wasn't that what I had drawn in my very first post?

 

[Audioguru]

Wasn't that what I had drawn in my very first post?

No. Your first post showed an NPN emitter-follower, not an NPN switch. Then the PNP transistor NEVER turns off.
If you change the NPN to be a switch and a higher supply voltage for the transistors then the logic will be backwards so another transistor (a third transistor) is needed.

 

[amzjl]

Can you further explain what you mean by the "logic is backwards?" Also, I am assuming since you showed it that way, I need to use two separate power supplies. I don't care either way, just wanted to know.

 

[Audioguru]

"The logic is backwards" means that the LEDs are turned on when you want them to be turned off and they are turned off when you want them to be turned on. The third transistor is an inverter.

You mentioned using a voltage for the LEDs that is too high for the Cmos so I added a 12V supply that can be used to drive strings of three LEDs in series.
A simple 6V zener diode powered from 12V through a series current-limiting resistor can be used to power the Cmos 555 from its 6V.

 

[amzjl]

Would the third transistor be another PNP and where would it go? Do I calculate the resistor leading into the zener just like the ones for my LED's by using the voltage drop and the max current the chip I select can handle?

 

[Audioguru]

Would the third transistor be another PNP and where would it go?

It inverts the output of the 555 like this:

Do I calculate the resistor leading into the zener just like the ones for my LED's by using the voltage drop and the max current the chip I select can handle?

The zener diode has a rated operating current. Since the supply current for the Cmos 555 is very low and the third transistor uses only a few mA then use a zener diode rated at 5mA and calculate the value of the resistor feeding it by using 3mA for the third transistor and 5mA for the zener so the current in the resistor is 8mA. The voltage across this resistor is 12V - 6V= 6V so Ohm's Law calculates its value to be 6V/8mA= 750 ohms. You can use 680 ohms if 750 is not available.