Hi all,

I used this calculator to get my 555 timer to get a high time of ~7sec and a low of ~1/2 a second, but I can't reverse it. I want ~1/2 second high, and ~7-15sec low.

I'm using 1000microFarad cap, R1 10k and R2 1k
http://www.ohmslawcalculator.com/555-astable-calculator

Thanks

 

Hi all,

I used this calculator to get my 555 timer to get a high time of ~7sec and a low of ~1/2 a second, but I can't reverse it. I want ~1/2 second high, and ~7-15sec low.

I'm using 1000microFarad cap, R1 10k and R2 1k
http://www.ohmslawcalculator.com/555-astable-calculator

Thanks

If you are just driving an LED, you can connect the anode to positive power and cathode to the 555 and the duty cycle inverts.

Otherwise, you'll need a diode to get less than 50% duty cycle. Search PWM 555 and you'll see the trick.

 

or use a transistor to invert the output of the 555-timer.

 

Or a diode(s) in the charge discharge side to get a duty cycle of less than 50%:



Source: AN170 (Philips)

John

 

or use a transistor to invert the output of the 555-timer.

I can't find a circuit like that, what do I ask google to find an example of a transistor changing the high and low?

I'm probably thinking about it wrong, but when I used a transistor before, sending positive to the base turned the NPN emitter and collector on.

 

I can't find a circuit like that, what do I ask google to find an example of a transistor changing the high and low?

I'm probably thinking about it wrong, but when I used a transistor before, sending positive to the base turned the NPN emitter and collector on.

Just use a "transistor as a switch". Essentially a class A amplifier (they invert the signal) because the output is attached to the positive voltage source. When high output of 555 goes into base of transistor, the transistor allows conduction and the output goes low.

 

but I can't reverse it.

change the topology: use 1/2 diodes or take the charge/discharge from the output pin.

 

I can't find a circuit like that, what do I ask google to find an example of a transistor changing the high and low?

I'm probably thinking about it wrong, but when I used a transistor before, sending positive to the base turned the NPN emitter and collector on.

There are lots of ways to use a transistor, if you post your current schematic we can give you better answers, probably something simpler than you have in mind.

Oft times it is easier for the expierenced here to solve your problem then to fix your solution.

 

To drive 1 LED, see post #2. To drive runway landing lights, add about a dozen really huge fat expensive power transistors.

Your application probably is somewhere between those two, but after 8 posts it still is a secret. Since there 100 ways to do what you are asking, how about a little more information so we can narrow it down.

ak

 

I can't find a circuit like that, what do I ask google to find an example of a transistor changing the high and low?

I'm probably thinking about it wrong, but when I used a transistor before, sending positive to the base turned the NPN emitter and collector on.

Google transistor inverter.

 

Use Pin 7 with pull up resistor.

 

As noted by others, we need more information about what you are trying to achieve to give better advice. What are you using the output of the 555 for? Do you have any logic in the rest of the circuit? It would be a shame to spend a lot of time coming up with a relatively complex solution if you have an unused NOT, NAND, or NOR gate sitting right next to your 555.

 

Thanks for all the replies everyone.

Ultimately, Mr. Chips solution worked for me. What I didn't understand about the NPN Tip 120 transistor was that you can run directly from the collector to positive with a resistor and run from the collector to your load to ground in parallel. That's not intuitive for a novice, but I think I'm starting to get it, and it's pretty elegant.

This is the schematic I used for my 555 timer LED flasher, but I adjusted the values of the capacitor / resistors and voltage. The only other difference was that I ran the output to the base of my a NPN transistor TIP 120. I can attempt to draw the circuit when I have some time for you, but this issue seems resolved. I will use a schematic when asking a question from now on. Apologies for wasting anyone's time.

 

@sirchuck
Note that the MrChips solution requires more power than the diode option. Not an issue with wall power but a big deal with batteries.

In the MrChips solution, either the current is powering the LED (when transistor is off and 555 is low). Or, current is flowing through the NPN transistor and turning off the LED by essentially shorting the current from the collector resistor to ground (giving a more convenient path to ground than through the LED).

With the diode option and no transistor to invert the output, current only flows while the 555 is high. This more than doubles your battery life since duty cycle is less than 50%.

 

Ultimately, Mr. Chips solution worked for me.

the simplest solution there would have been to put the led between the output pin and the rail. no other device needed at all.

 

the simplest solution there would have been to put the led between the output pin and the rail. no other device needed at all.

Exactly this. The 555 can pull down or sink a load just as well as it can source or drive a load. (Actually it is slightly better at sinking loads... less of a voltage drop across the device itself.)

 

@GopherT

The MrChips solution created the inversion I wanted, and I could see how to attach the components with the schematic. I currently don't understand how the diode option would work, or where exactly to put it to create the inversion I need. I can try to google 555 timer with diode later, if I find some examples I'll try it out on my breadboard.

@dannyf @ErnieM

"the simplest solution there would have been to put the led between the output pin and the rail. no other device needed at all."

I'm too much of a novice to see this one clearly. You are saying just putting the LED between the 5v rail and the 555 output without a resistor will invert the blinking? I can try it after work.

It does not make sense to me because on my circuit for ~7 seconds there is power coming out of the output pin #3, and then it's off for ~.5seconds. If I just put a LED... OH!

Ok that's cool!. It's still good to use a resistor in this method right? I mean I should be able to do this with any load not just a single LED right?

Haha, that just dawned on me. Damn it's so easy now, thanks for sticking with me, that must be about the most basic principle of electronics to have to try and explain. You made it so the only time there is a path to ground is when the 555 timer is in it's off position.

Stupid mental disconnects.

 

Haha, that just dawned on me.

I think the lesson learned here is not to ask people to help you go down a particular path. Instead, ask people to get you to your destination - as others may see shortcuts that you may not see.

the diode approach is also quite simple - see the one below. if your load is a small signal diode - as shown in your schematic, simply use a led in place of D2 and eliminate R3/D1.

 

Just use a "transistor as a switch". Essentially a class A amplifier

A nit, but a transistor used as a switch is Class D operation.
Class A is continuous operation in the linear mode.

 

here is the above circuit, using an led in lieu of D2. The plot is the current through the led.

the period is about 7 seconds. It kind of has an after-glow in that the led turns on instantly, and then glows to an off state gradually as the capacitor is being charged up. The current through D1, on the other hand, is instant on and instant off.

Whether that's important to you is hard to say.

The values will need to be experimented with different leds, obviously.