555: Difference between revisions
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The simple demonstration circuit depicted above right is probably the simplest external clock for the 555. Below we have hastily constructed a true precision “clock” using four off-the-shelf bipolar transistors and some auxiliary resistors. | The simple demonstration circuit depicted above right is probably the simplest external clock for the 555. Below we have hastily constructed a true precision “clock” using four off-the-shelf bipolar transistors and some auxiliary resistors. | ||
This circuit operates by charging capacitor C1 through resistor R5 only up to the trigger level of 2/3 the supplied voltage, and then discharging the same capacitor down to the threshold value of 1/3 the supplied voltage through resistor R2 only | This circuit operates by charging capacitor C1 through resistor R5 only up to the trigger level of 2/3 the supplied voltage, and then discharging the same capacitor down to the threshold value of 1/3 the supplied voltage through resistor R2 only. | ||
[[File:555-op-amp-ctl-clock.svg|frameless|none]] | [[File:555-op-amp-ctl-clock.svg|frameless|none]] | ||
Revision as of 15:05, 8 April 2026
The 555 is a ubiquitous "timer" chip used by electronics hobbyists and experimenters.
Clocking the 555
The simple demonstration circuit depicted above right is probably the simplest external clock for the 555. Below we have hastily constructed a true precision “clock” using four off-the-shelf bipolar transistors and some auxiliary resistors.
This circuit operates by charging capacitor C1 through resistor R5 only up to the trigger level of 2/3 the supplied voltage, and then discharging the same capacitor down to the threshold value of 1/3 the supplied voltage through resistor R2 only.
Notes
Transistors Q1 and Q4 operating in parallel appear redundant, and it would seem desirable to simplify the circuit to eliminate one of them. We have not determined the best way to do this with a simulated design, however.
R9 is an arbitrary load resistor for the output, probably less than desirable.
We have produced a simulated oscilloscope trace using Ngspice [1] on KiCad [2].
- ↑ ngspice - open source spice simulator https://ngspice.sourceforge.io/
- ↑ KiCad: A Cross Platform and Open Source PCB Design Suite https://www.kicad.org/
