Basic 555 questions

Discussion in 'The Projects Forum' started by JLother, Mar 12, 2014.

1. JLother Thread Starter New Member

Jan 16, 2014
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Moderator note: this thread has been split from 5 sec delay ON timer.

Can anyone explain what the resistor and capacitor on the top schematic on the left side of the 555 are doing and what the resistors coming in and out of the transistors are for? I want to use this method to power on some light panels but I'm only using 6 volts. I was not sure what was necessary or not. I was assuming the resistor and capacitor were there to short out the trigger pin and start the timer but there isn't any values. Am I correct? Any information would be good.

Last edited by a moderator: Mar 12, 2014
2. #12 Expert

Nov 30, 2010
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I really think you will do better with your own thread because your questions are many and your voltage is very different. I have asked a moderator to give you your own thread.

3. Wendy Moderator

Mar 24, 2008
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The parts on the left are to trigger the monostable on power up, and will work at no other time.

4. #12 Expert

Nov 30, 2010
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The resistors on the transistors are there to limit current so the transistors don't burn up. Using 6 volts instead of 28 volts will change the required resistance.
Please describe the voltage and current the light panels need.
Please describe how much delay you want when you apply power.

5. JLother Thread Starter New Member

Jan 16, 2014
10
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My lights are 3.2 volts. The 555 I'm want to use only requires 1.5 volts. The delay I want is 3 seconds and I believe I have already figured out the timing components. Do I need resistors with the transistors with these voltages? I was going to try using one timing circuit for all 30 LEDs and using a separate timing circuits for each panel of 6. What will change between the two? All lights are ran in parallel. Also what capacitor value should I use coming off the trigger pin to start the timing?

6. #12 Expert

Nov 30, 2010
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Yeah...love your enthusiasm but...
I can't find a 555 timer that runs on 1.5 volts. Try using 6 volts on this one. It will make life easier. And, LEDs are not light bulbs. They are diodes and need a resistor for each series string. Don't put LEDs in parallel. They will fight each other for the current and burn out, one by one. The trigger circuit can be made with .1uf capacitors and 10k resistors. And, again, yes you need resistors for the transistors. They are a sort of dual junction diode and have no sense about how much current to use.

Here, look at this and follow the link on the second page to see a dozen ways to connect LEDs. This will hurt less if you get the basics figured out.

7. JLother Thread Starter New Member

Jan 16, 2014
10
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Its called a TS555CN and it does exist. It used specifically for battery powered applications. Now I understand I can find out from a transistors data sheet how much current it can handle but how can I figure out the supply coming from the chip to the npn's base and from there to the pnp's base?

8. ericgibbs AAC Fanatic!

Jan 29, 2010
2,504
380
hi,
This is a clip from the TS555CN datasheet, at 2V, which is the minimum specified operating voltage.

E

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9. #12 Expert

Nov 30, 2010
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When using transistors as switches, the method is to give the base of the transistor 1/10th of the needed collector current. This makes the transistors use up very little voltage so as to become almost insignificant about wasting power. If you can't get 10% of the needed current out of the timer chip, you have one transistor drive yet another transistor to get the current ability to increase by powers of 10 for each transistor.

Darlington transistors and Sziklai pairs were invented to do this stacking function in what externally looks like a single transistor.

If you feed your timer 6 volts, use the 5 volt chart to see 2 ma available when the output is high and 8 ma available when the output is low. 2 ma driving one transistor will get you 20 ma through the collector. If you add another stage, be it npn or pnp, that 20 ma can be amplified to 200 ma.

Last edited: Mar 14, 2014
10. JLother Thread Starter New Member

Jan 16, 2014
10
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OK, here is the data sheet for the part I want to use. Someone explain if I can do what I'm trying to accomplish with this item. Keep in mind that I want to run enough power to 30 LEDs and a separate system using only 6 LEDs while using transistors as a switch. Can this item do both?

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11. #12 Expert

Nov 30, 2010
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That's an LMC555
Yes, one for the 30 LED project and one for the 6 LED project.
If you want to minimize parts, there is a dual 555 called a 556.
If you want higher current out of the chip, the LM555 or NE555 version can do about 200 ma.

12. ericgibbs AAC Fanatic!

Jan 29, 2010
2,504
380
hi,
This image is a clip from your latest d/s.

As #12 states you will need a transistor pair, the Vout of the LMC555 is typically only +1.25V when sourcing current, a Darlington Base will require at least 1.3V.

As you are driving LED's you must have a higher voltage available, so why dont you use a that higher voltage to power the 555.?

E.

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shortbus likes this.
13. JLother Thread Starter New Member

Jan 16, 2014
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The lower voltage chip has a much wider temperature range that I want. I don't really mind using higher voltage but I'm trying to keep the entire package, battery pack and circuitry as small as possible. I also would like to use common battery sizes like AA and AAA rather than CR123 to gain the required voltage. Fix me up guys. If you could include where you found the part so I can purchase. I need the chip, transistors, capacitors, and resistors for the timing portion of my circuit other than the actual timing capacitor and resistor.

14. #12 Expert

Nov 30, 2010
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Here's a start. I order parts from www.mouser.com
You need to check on how long your battery can last at 6/10 of an amp.

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15. JLother Thread Starter New Member

Jan 16, 2014
10
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I have figured out the battery problem. If the lower voltage 555 works, I have one power supply and if it doesn't, another. The timing circuit it what is holding me up. So the TS555CN doesn't have enough power coming from the output to switch an NPN on?

16. #12 Expert

Nov 30, 2010
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Sure it does, just add more transistors. I spoke of this in post #9

17. JLother Thread Starter New Member

Jan 16, 2014
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Would this be a better alternative and what values do I need to figure out before I go hooking power to it? Could I use this with only two transistors (NPN and PNP) like the original schematic showed, where one keeps the other off for the timing duration and then allows power through it once the timing is over?

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18. #12 Expert

Nov 30, 2010
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With a 6 volt supply, this chip can provide 1 ma at 5.1 volts.
Put a 4.7 k resistor on the output and have that feed the base of a transistor. That transistor can then drive 10 ma into a second transistor. That transistor can then drive the 60 ma you need to drive the 3rd transistor, which can drive the 600 ma needed for 30 LEDs.

If you want to give the chip 2 volts, you will need 4 transistors to arrive at the required current but the first resistor will be 3.3k ohms.

19. JLother Thread Starter New Member

Jan 16, 2014
10
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Is it possible to create the circuit with just two transistors with either the 1.5 volt TS555CN or the Texas Instrument chip I showed in my last post if I only use 6 LEDs?

20. #12 Expert

Nov 30, 2010
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6 LEDs @ .02 amps each = .12A
.12A needed/.0003A available = gain of 400
That will need 3 transistors.

Here's a drawing of a circuit that looks like it only has 2 transistors, but it really has the quality of 3.

• CMOS 555 to LEDs.png
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Last edited: Mar 16, 2014