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I am building a 555 astable timer circuit for a course which must match the BPM of a song. The song has a BPM of 104, which requires a timer frequency of roughly 1.733 Hz.

The circuit looks as follows: enter image description here

(the green wires go off to the left onto another breadboard, which looks as follows:)

enter image description here

It is hard to see, but the 0.01µF capacitor goes to pin 5 of the 555 timer.

The values used in the circuit as follows (measured using a LCR meter): R1 = 5.5kΩ R2 = 38.5kΩ Ctotal = 10.02 µF

The problem is that the circuit produces a frequency of roughly 1.5, far off the from the desired/predicted frequency of 1.733. 5V Power is being supplied to both rails, the chip has been replaced, but to no avail. Here is a picture of the simulated circuit in LTspice: enter image description here

Is this due to some limits of the 555 timer at frequencies nearing 1Hz or due to some miswiring on my part?

schematic

simulate this circuit – Schematic created using CircuitLab

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    \$\begingroup\$ It is hard to see, but the 0.01µF capacitor goes to pin 5 of the 555 timer. - yes, that's why we demand schematic diagrams (use the diode/capacitor/resistor button on the toolbar) rather than photos of assembled circuits. \$\endgroup\$ – Neil_UK Oct 17 '18 at 18:09
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    \$\begingroup\$ The axial electrolytic capacitors appear to say 1 uF 50 V, and they are in series. How do they add up to 10 uF, am I looking at the wrong picture? Also, 10.02 uF is very accurate for an electrolytic capacitor, 0.2%, they're typically ±20%, are you sure the measurement was OK? \$\endgroup\$ – tomnexus Oct 17 '18 at 18:51
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    \$\begingroup\$ Am still suspicious of those electrolytics. They are "not very good" in many ways. It is possible that your LCR meter measures C in a way that differs from the way it is used by the LM555. \$\endgroup\$ – glen_geek Oct 17 '18 at 19:26
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    \$\begingroup\$ Expecting such a precise value of f when using components with at least 10% tolerance is not practical. Make your frequency dependent from a potentiometer and calibrate it afterwards. But be warned, even with this trick, your frequency will vary with temperature. \$\endgroup\$ – Sredni Vashtar Oct 17 '18 at 20:16
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    \$\begingroup\$ "The 10µF is not very good and has a capacitance of <9.5µF". What's not good about that? Electrolytic caps are typically specified with a pretty broad tolerance. I suspect you're well within specs on that one. \$\endgroup\$ – TimWescott Oct 17 '18 at 21:37
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As practically everyone has commented, electrolytic capacitors are not usually chosen for precise timing applications. CDE capacitors have typical capacitance tolerances of ±20%, –10% +50%, and –10% +75%. So your circuit is most likely working perfectly, but the value of the capacitor is not known.

However, since you have the circuit built and running, you can use it to estimate the resistor values required to make the circuit operate at your desired frequency. If you are at 1.5 Hz, you are at 1.5/1.733 or 86.6% of your target frequency. You can achieve 1.733 Hz by multiplying the values of the two timing resistors by 0.866, which would yield (in this case) 4.76K and 33.3K. You can do the math and put in parallel resistors to achieve this result. Even if you were to spend the money and get precision resistors and capacitors, the 555 timers will vary a couple of percent from chip to chip, so that in practice it is common to build a circuit that gives a too-low frequency (just as you have done) and leave pads to add parallel resistors to the timing resistors which are selected to compensate for component errors.

Good luck!

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  • \$\begingroup\$ Either putting several resitors in parallel or just use a potentiometer. Then you are able to exactly trim your frequency. \$\endgroup\$ – jusaca Mar 14 at 12:16
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You could replace the 2 resistors with trim pots and dial in the values better.

https://www.digikey.com/products/en/potentiometers-variable-resistors/trimmer-potentiometers/80?k=trim+pot&k=&pkeyword=trim+pot&sv=0&pv69=80&sf=1&FV=ffe00050&quantity=&ColumnSort=0&page=1&pageSize=25

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