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I'm trying to use a NE555D to achieve a 500ms on, 1500ms off, duty cycle. The exact timing is not that important(+-20%).

But my problem is that when the temperature changes it greatly effects the blinking rate. blinks faster when it is hot and stops blinking when cold. How can I improve this? This IR beacon is something you wear so it needs to handle the temperature differences.

The values of R1 and R2 are tweaked until roughly 500ms on 1500ms of duty cycle is achieved (+-20%).

edit Supply V is 3.3, all the resistors are metal film and capacitor is X5R

thank you for all the feedback so far! Im doing my best to understand it all.

Example pictures would be greatly appreciated.

Low frequency asymmetrical led blinking circuit

OpticalOpal
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    Your schematic has no very important supply voltage. Remove D3. Also, the old NE555 draws a power supply current pulse of almost 400mA (!) each time it switches and without a decoupling capacitor to power it during the 400mA, then its supply voltage through the 50 ohms resistor drops 20V! – Audioguru Aug 15 '23 at 14:47
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    "... exact timing ... greatly effects ...". Please _edit your question_ with information _in engineering units_ about what timing variation is allowable, and what timing variation you see. **Your** "exact timing" may be **my** "hopelessly loose" timing; **my** "greatly" may be **your** "hardly noticeable". – TimWescott Aug 15 '23 at 15:02
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    I agree with @Audioguru that you need decoupling. No matter what, you should use a 100nF cap from the 555's power pin to its ground pin, located as close to the package as practical (snugged up against one end of the package or the other is good). If you're using that older style 555 to which they refer, and if you really need R9 there, to isolate the 555 from the rest of the world, then you should use a 10-470uF cap from the 555's power to ground. If you make sure to use a new CMOS 555 (and you have _read the datasheet carefully_) then you may be able to get away without it. – TimWescott Aug 15 '23 at 15:05

2 Answers2

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All four of the components in the timing portion of your circuit are temperature-dependent: R1, R2, C2, D3. The change in forward voltage of D3 probably has a negligible effect on timing. R2 is 90.9k, which suggests 1% tolerance, which in turn suggests metal film, so the temperature coefficient is probably acceptably low - perhaps 100ppm/C. If R1 is carbon film, then the tempco will be higher. The big issue is that 10uF capacitor; if it's electrolytic, then you might be losing 10s of % over the temperature range.

I would probably ditch the diode as @Audioguru says (and high side switch with PNPs if you need the duty cycle), add in a decoupling capacitor on the 555 and some bulk capacitance close to the LEDs, make sure the timing resistors are metal film, and use a more temperature-stable capacitor for C2, maybe X7R, X5R, or tantalum. If that still doesn't get you where you need to go, then I suggest a cheap microprocessor with a few lines of code and an external oscillator which you can get in any level of temperature stability you desire.

vir
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  • Thank you for the answer! I will look in to the component materials. I have considered a microprocessor but i have not found a easy enough guide for me to follow with my limited knowledge. – OpticalOpal Aug 16 '23 at 07:20
  • Turns out all the resistors are metal film and capacitor is X5R. so just changing the components is probably not an option. – OpticalOpal Aug 16 '23 at 07:55
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the cause is the almost certainly the capacitor. You need capacitor that is not also working as a thermometer. one that is temperature stable.

If you use larger resistors and and a smaller capacitor (like 100nF), instead, this will make it easier to use temperature stable capacitors like PPS or PP film capacitors. 5M resistors should be no problem as the CMOS based 555 has high impedance inputs for trigger and threshold. this may mean that you need to clean the circuit board more. C0G is also an option only moderatly expensive at 100nF but prohibitive at 10uF

Jasen Слава Україні
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