In the last blog, I used a 555 timer to make a simple oscillator. It used an
additional transistor, as a current source for the capacitor charging, in place
of the resistor that would usually be used. Although the blog focused on the
linear nature of the ramp and the resulting sawtooth waveform, my real interest
was slightly different as I wanted to use it as a VCO (Voltage Controlled
Oscillator).
Here I'm taking that circuit and adapting it so that I can control the current.
I need the control input to accept voltages in the range of 0-3.3V and to be a
very high impedance as it will be controlled by the voltage across a capacitor
(hopefully without discharging it). This is the circuit I ended up with.
On the right we still have the transistor and emitter resistor that function as
a current source, but now, instead of the base voltage being fixed by a
potential divider, it can be adjusted by the op amp. To the left, I have a
mirror image of the transistor and emitter resistor. Although the transistors
won't match exactly, the currents in the two 620 resistors will be approximately
the same. The current in the left 620 flows down through the transistor and
mostly out of the collector where it develops a voltage across the 3k3 resistor
that can be read by the op amp. That then gives voltage feedback that's
equivalent to the current that's flowing down into the capacitor. Final step is
for the op amp to compare that with the control voltage going in and drive the
bases in such a way that the two match. Op amps are good at that kind of thing,
particularly when you get the two inputs the right way round! (Yes, initially I
managed to overlook the inversion in the feedback path given by the transistor.)
It's not the most inspired piece of design ever and you probably wouldn't want
to use it for instrumentation (V -> f converter), but for what I want it will
suffice, though there is one snag with it which I will need to work around.
Here's a graph of the resulting frequency for different control voltages. 
It's fairly linear, though there's a slight offset (it doesn't look like it
passes through the origin: thinking back, it's possible I had a 'scope probe on
the timing capacitor stealing some of the current, though a bipolar 555 will be
biasing it a little as well). The range is also somewhat limited. At the lower
end it becomes increasingly jittery when the frequency gets down to a few
hundred Hertz, presumably because of the low currents involved, so maybe two
decades at best without some more work.
If I tell you what the snag is - the oscillator stops with a control voltage of
0V, meaning there will no longer be edges in the output waveform - you may be
able to guess where I'm going for part three.
Next blog: 555 Phase-Locked Loop (PLL)
If you found this interesting and would like to see more blogs I've written, a
list can be found here: jc2048 blog index
References
[1] https://www.st.com/en/clocks-and-timers/ne555.html
[2] https://www.ti.com/lit/ds/symlink/ne555.pdf
Top Comments