Light NeoPixel with 555 Timer IC (Part II)
To continue the discussion from Part I, the design goal is to generate BIT-1 for NeoPixel with 555 IC.
The following is another 555 astable circuit uses only one pair of RC, and it outputs equal high time and low time.
It can be analyzed using that RC step response equation (see Part I of this article).
Although VF, VT and V0 take different values for charging and discharging of the capacitor, the left hand side end up being the same value:
So the pulse width Δt = ln2 * R1*C1 = 0.69 * τ
With R=1.8k Ω and C= 330pF chosen, τ = 0.6μs and Δt = 0.4μs.
That 0.4μs matches the low time for a BIT-1 of the WS2812B NeoPixel controller.
So we just need to generate the 0.8μs high time, in order to send BIT-1 to NeoPixel and light it up.
One way to get a longer time pulse is to add another RC time constant τ, as Julian did. (discussed in Part I of this article)
But there is another way. That is to use the “control” pin of 555 to “modulate” the pulse width. The following circuit is simulated with a second voltage source V2 on the “control” (CV) pin of 555 IC.
Again the nice RC step response equation can be used to calculate its pulse high time and low time.
For the low time (i.e. capacitor discharge to 1/2Vc)
Vc is the voltage on the “CV” control pin. 1/2Vc is due to the internal resistive divider.
On the left hand side, Vc gets cancelled out. So the low time is independent of the control voltage, and the result is the same as before: Δt = ln2 * R1*C1 = 0.69 * τ = 0.4μs. Good news!
Now we can focus on adjusting for the high time. The high time, that is the capacitor gets charged from 1/2Vc to Vc, can be calculated as