Hi,
I am working on a project that uses a 555 timer with a tactile push button to create an on/off toggle.
The toggle then drives another 555 timer set up in astable mode for a 1Hz pulse. This 1Hz pulse then drives another 555 timer set up in astable mode at 8Hz. That in turn drives a NPN transistor (2N4401) that drives 6 LED's.

The idea is that it strobes the LEDs for about 1 second on an off.

I measured the current when both toggled on and off. When on it pulls about 20mA, which seems fine. But when it is off it pulls about 4mA, which is a problem. I'm still pretty new to this, so I can't understand why it's pulling current when it's supposed to be off. I think it might have something to do with the wiring of the transistors that are driving each of the 555 astable circuits, but I really don't know.

The other thing is that I can drive the toggle from 5V, but the 2 astables won't run on 5V, but they do run on 9V. Is there a way to get change the wiring to get the whole circuit to run on 5V?

I've attached a image of the schematic I am using

 

The 555's take several mA when idle as shown below:


The LMC555 CMOS version takes much less current (below) so you may want to use those if you are concerned about quiescent bias current:

 

When you try to run your circuit from 5 volts, the final NE555 timer can't run on the small remaining voltage available...
Why? The first NE555 timer (IC4) only puts out 5 volts minus about 1.5 volts. The first transistor drops another 0.7 volts from that giving about 2.8 volts to the second timer (U5). The same losses happen in the second timer circuit leaving no voltage to run the last timer much less the LED's.

I don't have time right now to supply a solution to the problem. Hopefully someone else such as @crutschow will join in.

 

The eventual plan is to run this circuit from a LiPo battery, currently it's running from a 9V battery.
So I figure I will have to use a buck boost regulator. Since I'm fairly new to this I would prefer to use an off the shelf solution, at least to start with.
All of the buck boost regulators I've seen so far put out 5V from the 3.7V LiPo input , hence my original desire to use 5V supply.

Can anyone please suggest a way to power this circuit via a LiPo battery and buck boost, or perhaps something else?

 

With careful design you may not need to boost the battery voltage. By the way, what are the LED's you are using? What is their color?

 

Is there a way to get change the wiring to get the whole circuit to run on 5V?

Delete Q1 and R11. Connect IC4 pin 3 to IC5 pin 4. Same between IC5 and IC6. For IC5 and IC6, connect the Vcc pins, R13, and R16 directly to Vcc.

Also, the total LED current is less than 130 mA at 9 V. The 555 can drive this directly, eliminating R17 and Q3.

Also also, you can replace the three 555's with one CD40106 hex inverter, saving five decoupling capacitors and some other stuff. The 40106 would need an external LED drive transistor, such as a 2N7000. 11 discrete components vs. 21.

ak

 

With careful design you may not need to boost the battery voltage. By the way, what are the LED's you are using? What is their color?

I'm currently using green LEDs because that's all I had on hand. However I plan to use yellow LEDs.
Here is their datasheet: https://media.digikey.com/pdf/Data Sheets/Lite-On PDFs/LTL2R3KYD-EM.pdf

I have ordered some 556 CMOS timers. Here is their datasheet: http://www.ti.com/lit/ds/symlink/tlc556.pdf
From what I can tell, page 5 shows the Supply Current as 130uA.
http://www.ti.com/lit/ds/symlink/tlc556.pdf
The whole idea is for the LEDs to be bicycle safety lights so that car drivers can see the cyclist from the side at night.

The plan is to have a solar panel trickle charging a LiPo battery during the day. Here is the solar LiPo charger datasheet: https://media.digikey.com/pdf/Data Sheets/DFRobot PDFs/DFR0264_Web.pdf
Then at night time the cyclist can press the tactile button to turn on the strobing LEDs.
When the cyclist is finished their ride they can then use the tactile switch to turn off the LEDs.

If it's possible to not use the buck boost converters that would be ideal.

 

Do what AK suggested and, if you want to minimize quiescent power, use the CMOS LMC555 timers.

How long do you want this to run on one set of batteries?

 

Do what AK suggested and, if you want to minimize quiescent power, use the CMOS LMC555 timers.

How long do you want this to run on one set of batteries?

I've ordered a 3.7V 850MAH LiPo battery. If I could get 3-5 hours of run time from a single charge I would be very happy. Then the battery could trickle charge via the solar panel during the day ready for use again.

From what I understand reading the datasheet, each 556 CMOS timer has a Supply Current of 15mA (each for a total of 3). Then there is supply current for each LED. If I use 10mA per LED, then that's 60mA.
So if I understand this correctly my total current would be 105mA. So then for an hour of use, thats 105mAh. So from a 850mAh battery I should be getting 850mAh/105mAh = 8.1 hours.
Does this sound right, or am I making a mistake?

 

Here is an all CMOS solution with very low standby current.

ak

 

...each 556 CMOS timer....

A 556 is a dual bipolar timer, not a CMOS device.
The LMC555 is a CMOS device with much lower supply current.

From what I understand reading the datasheet, each 556 CMOS timer has a Supply Current of 15mA (each for a total of 3). Then there is supply current for each LED. If I use 10mA per LED, then that's 60mA.

The LEDs are only on about half the time, right(?), so their average current would be 30mA

 

Consider replacing the toggle flipflop with a simple on-off switch. The ff adds to the overall project complexity with no real benefit.

ak

 

Delete Q1 and R11. Connect IC4 pin 3 to IC5 pin 4. Same between IC5 and IC6. For IC5 and IC6, connect the Vcc pins, R13, and R16 directly to Vcc.

Also, the total LED current is less than 130 mA at 9 V. The 555 can drive this directly, eliminating R17 and Q3.

Also also, you can replace the three 555's with one CD40106 hex inverter, saving five decoupling capacitors and some other stuff. The 40106 would need an external LED drive transistor, such as a 2N7000. 11 discrete components vs. 21.

ak

When I get home I'll try driving the 555's from the previous 555 stages.

For the CD40106 I found this: http://www.talkingelectronics.com/pay/BEC-2/Page49.html
which when I get home I'll have a look if I have any.

Here is an all CMOS solution with very low standby current.

Would I be correct to say that this can be run from 5V, provided I used the appropriate resistors for the LEDs?

 

A 556 is a dual bipolar timer, not a CMOS device.
The LMC555 is a CMOS device with much lower supply current.

The TLC555 is made with a LinCMOS process according to the datasheet. I'm not sure if that means it is a CMOS device or not.
http://www.ti.com/lit/ds/symlink/tlc555.pdf

The LEDs are only on about half the time, right(?), so their average current would be 30mA

Good point

Consider replacing the toggle flipflop with a simple on-off switch. The ff adds to the overall project complexity with no real benefit.

ak

Do you mean a SPST switch?
I much prefer a tactile button switch for asthetics.

 

The TLC555 is made with a LinCMOS process according to the datasheet. I'm not sure if that means it is a CMOS device or not.

Yes LinCMOS is TI's linear CMOS process.
Notice its low supply current.

 

Yes LinCMOS is TI's linear CMOS process.
Notice its low supply current.

Oh good, I saw that the current was low, just wanted to make sure that I wasn't misunderstanding the datasheet.
Thank you!

 

For the CD40106 I found this: http://www.talkingelectronics.com/pay/BEC-2/Page49.html
which when I get home I'll have a look if I have any.

THe main difference between a 74C14 and a CD40106 is that the 14 is a 5 V-only device, while the 40106 is 3 V - 18 V.

Would I be correct to say that this can be run from 5V, provided I used the appropriate resistors for the LEDs?

Yes, the circuit I posted will run on 5 V, 0r 3.6 V.

ak

 

Do you mean a SPST switch?
I much prefer a tactile button switch for asthetics.

What about an alternate-action pushbutton switch?

ak

 

THe main difference between a 74C14 and a CD40106 is that the 14 is a 5 V-only device, while the 40106 is 3 V - 18 V.

Yes, the circuit I posted will run on 5 V, 0r 3.6 V.

ak

Sorry to inundate you with questions, does that mean the circuit would work with a changing voltage such as one from a LiPo battery?

 

does that mean the circuit would work with a changing voltage such as one from a LiPo battery?

Slowly changing, yes. The datasheet has the operating voltage conditions. And the changing operating voltage should not affect the oscillator frequencies very much. The two hysteresis transition levels are set as percentages of Vdd, not absolute voltage values, so they should track varying Vdd well.

ak