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I am currently looking at generating a signal to blink a led with a 10ms duration every 2 seconds. I was thinking to use a 555 timer to do so, but I don't see how to change the duty cycle to make that off time is different than on time.

I want to avoid the use of an MCU if possible to save energy.

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  • \$\begingroup\$ I think a astable multivibrator example circuit online will tell you how to make that exactly. \$\endgroup\$ – Bradman175 Aug 13 '16 at 8:46
  • \$\begingroup\$ @Bradman175, i was thinking to use : ti.com/lit/ds/symlink/lmc555.pdf \$\endgroup\$ – chris Aug 13 '16 at 8:51
  • \$\begingroup\$ Page 10 says it all... \$\endgroup\$ – Bradman175 Aug 13 '16 at 8:52
  • \$\begingroup\$ @Bradman175 , I just saw it. I didn't know this was called Astable operation. Thanks \$\endgroup\$ – chris Aug 13 '16 at 8:56
  • \$\begingroup\$ well it is actually important to know that. At least you know now ;) \$\endgroup\$ – Bradman175 Aug 13 '16 at 8:57
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So this is also from \$2V\$ to \$3V\$. Is this another question related to your MSP430 question, earlier, called "connect a 5mm led to a gpio without transistor"? Is this just another way of addressing your earlier question? If so, you really are better off just getting a high efficiency low current LED and doing this in software. You'll get precision control of timing and it's cheap and easy. Plus, using the existing MSP430 for this (which has fabulous sleep modes and very very fast re-start from sleep capability) then the power consumption is really at a minimum, too. In fact, even if you had to add another MSP430 for ONLY this purpose, it would still be a very very low energy alternative. Those things sleep on sub-microamp draw with a timer running and can fire up to full speed in about a microsecond. I can't see why it wouldn't be a solution here.

Honestly, I don't know why you don't stay with that solution. But I'm going to assume this is for a different purpose than that one.

Before moving on, the MSP430G2210 is an 8-pin part that costs about $1 in ones. It includes an internal VLO that, in LPM3, draws about \$0.5\mu A\$ and can wake up and have the DCO running in about \$1\mu s\$. (You may not even need to bother with the DCO, but the VLO might be \$250\mu s\$ cycles and the DCO can be much much faster and get the few instructions needed done in much shorter time, so it might be worth it to start the DCO anyway.) So you turn on the LED, go to sleep, turn off the LED, go to sleep. Etc. Assuming you fire up to \$12MHz\$ in \$1\mu s\$ and run for another \$4\mu s\$ before going back to sleep (draw about \$3mA\$), that's \$5\mu s\$ every \$2 s\$ at \$3mA\$ and the rest at \$0.5\mu A\$. Add to that, let's say 10mA for the LED during the \$10ms\$ period. Average of \$100.5\mu A\$ draw total. That's basically just the LED itself (\$100\mu A\$ average) with the MSP430 not counting for anything. And that's better than an LM3909 will do. Speaking of which:

There is an IC called an LM3909 which, if you can get one, would probably solve the need. They run off of as little as about \$1.2V\$ and they work fine running on up to \$6V\$. So that covers your range of \$2V\$ to \$3V\$. They will use more than \$1 mA\$ to get the job done, though. You don't even say how many \$mA\$ you want to drive through the LED during that 10mS period, so I will take some freedom there, too. (The LM3909 delivers a high initial current that will probably be more than \$20mA\$ and then let's that drop down to about \$20mA\$ over a period of \$5mS\$, for one combination of values I tested. Which might be fine for you. But who knows? You didn't say what you can accept.)

If you are serious about a completely separate circuit for blinking the LED and truly don't want to use the LM3909 for other reasons, then here is a workable LM3909 in discrete form:

schematic

simulate this circuit – Schematic created using CircuitLab

Again, the current consumption of a dedicated MSP430 is less.

You could also consider this circuit:

schematic

simulate this circuit

It's also extremely low power. It will not work at \$2V\$, though. I think you may be able to operate down to somewhere around \$2.5V\$, perhaps? Definitely at \$3V\$. So it may not be a fit. It was something I was considering attaching to the phone line to monitor the activity, as it converts voltage to frequency pretty well and with different values for \$R_1\$ and \$C_1\$ wouldn't exceed the phone company's maximum on-hook impedance for a phone attachment.

But you really do need to specify a LOT more than you do when you write. In general, you aren't disclosing yourself very well nor discussing a range of acceptable behaviors or constraints.

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  • \$\begingroup\$ we are actually wondering if we would choose the MSP configuration or a "without MSP" circuit. Advantage of MSP is we can do whatever we want with logic and blinking those led the way we want easily. With analog solution this is more complicated but it is also potentially less power hungry and probably cheaper. so we are currently wondering what we shoudl do \$\endgroup\$ – chris Aug 17 '16 at 6:13
  • \$\begingroup\$ Thank you very much for your feedback, it is very usefull. We will go with the MSP 430 solution then. \$\endgroup\$ – chris Aug 17 '16 at 6:16
  • \$\begingroup\$ Ialso take note of your last note about disclosing more details. I will anyway probably submit the circuit in a short term just for reveiw. \$\endgroup\$ – chris Aug 17 '16 at 6:16
  • \$\begingroup\$ Okay. If you need explanations of the discrete circuits, that can be provided I suppose. But my own preference is the MSP430 approach. It's lots more flexible, provides an avenue to add other control features should you later need them, and may take up less room besides. \$\endgroup\$ – jonk Aug 17 '16 at 6:32
  • \$\begingroup\$ We have decided to go with the MSP430FR2311IPW20R finally (we have a bit some inventory and we have used it on another project). What we try to do is simply to display fuel gauge of this battery and to triger some LED when some charge level are reached. So we have a LTC2935CTS8-3 to gauge battery we use logic to make blink led based on the two output logic of this chip. \$\endgroup\$ – chris Aug 17 '16 at 6:37
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enter image description here

Figure 1. 555 timer with adjustable duty cycle.

To adjust the duty cycle you create alternate charge and discharge paths for the timing capacitor using diodes. This example uses a potentiometer to adjust. You can leave it and R3 out if you calculate the required values.

See LearnAbout-Electronics.


As @DanielTork suggests you can do the same thing by adjusting the charge (R1) and discharge (R2) resistors.

enter image description here

Figure 2. A CMOS 555 will consume much less power. Source: All About Circuits.

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  • \$\begingroup\$ 2 fast 4 me, transistor. \$\endgroup\$ – Bradman175 Aug 13 '16 at 8:46
  • \$\begingroup\$ But isn't it true that you can also vary the duty cycle by changing the values of the capacitor and the resistors in the conventional astable setup? \$\endgroup\$ – Daniel Tork Aug 13 '16 at 9:55
  • \$\begingroup\$ Yes. I didn't do the maths to see if you can get 1:100 ratio that the OP requires. I'm not an 555 fan but the OP asked ... \$\endgroup\$ – Transistor Aug 13 '16 at 10:03
  • \$\begingroup\$ Ok,I didn't mean to criticize you.I was just checking. \$\endgroup\$ – Daniel Tork Aug 13 '16 at 11:44
  • \$\begingroup\$ Your comment was fine. Please feel free to copy and improve. \$\endgroup\$ – Transistor Aug 13 '16 at 11:46
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A low power 555 (from what I can tell) will consume about 200 uA. The LED (20 mA) with a 200:1 duty cycle will consume about 100 uA.

I want to avoid the use of mcu if possible to save energy?

If you want to save energy think about choosing a non-555 solution. Off the top of my head, there are very low power op-amps such as the OPA333 (TI) that consume about 20 uA. With appropriate high value resistors and capacitors you could fabricate an asymmetrical duty cycle schmitt trigger relaxation oscillator: -

enter image description here

High value resistors MUST be used to avoid too much energy loss and I'm thinking that a basic feedback resistor to the cap would be 10 Mohms with a diode and 200k series resistor across it to give a duty cycle of 200:1.

Op-amp input leakage currents are sub nano amps so 10 Mohms should not be a problem. On the positive feedback side a 10 M resistor should also be used with a couple of 1 Mohm resistors as shown although you might get away with 4M7 resistors.

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  • \$\begingroup\$ Typically what I am looking to do in our case is a led indicator such as in smoke alarm powered by battery \$\endgroup\$ – chris Aug 13 '16 at 9:53
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    \$\begingroup\$ @chris well the battery supply of 9V (if that's what you use) will be a problem to an OPA333 but, my answer is ONLY an indication of what is possible. A 555 will take more power over the lifetime of the battery than the flashing LED will unless someone can find a really lean and mean 555. \$\endgroup\$ – Andy aka Aug 13 '16 at 9:57
  • \$\begingroup\$ no we use a CR2, which the vfoltage goes from 3V to 2v \$\endgroup\$ – chris Aug 13 '16 at 10:02
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    \$\begingroup\$ That's probably on the low limit of a 555. \$\endgroup\$ – Andy aka Aug 13 '16 at 10:08

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