# Interesting 7555 circuit.

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#### Hero999

##### Banned
Can anyone guess what it does?

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#### ljcox

##### Well-Known Member
It's an oscillator.

When the output goes high, the voltage at pins 2 & 6 rises at the rate set by the L/R time constant. When it reaches the upper threshold, the output goes low so the voltage at 2 & 6 decays. When it reaches the lower threshold the output goes high again.

#### Hero999

##### Banned
I was bord and started plaing around with components.

I am aware that RC oscillators exist but I've never seen any LR oscillators so I decided to design one. However, I don't see how this is any use, other than for determining the value of a large value inductor using a frequency counter.

#### ljcox

##### Well-Known Member
Yes, capacitors are cheaper than inductors.

Yes, it could be used as a crude inductance measuring set.

##### New Member
Sorry for posting on a tread that low, but did you try this circuit?
I am asking this because I am at the start of various SMPS experiments
and I need to know how an inductor behaves when loaded with large currents.
Any idea if output frequency/period will be linearly proportional to inductor value?
Regards

#### Hero999

##### Banned
It works, there are much better ways to build an oscillator though.

##### New Member
Did you take a look at the voltage waveforms?
I tried simulating it in LTSPICE but voltage on 2+6 pin node swings from gnd to supply voltage, while in a normal 555 circuit it should swing from 33% to 66% of Vcc.
I was thinking about measuring an inductance, not about constructing a generic oscillator with it.
Regards:
Dimitar

#### Hero999

##### Banned
I would have thought pin 6 would swing from 33% to 66%.

Going from the 7555's datasheet the frequency should be approximately equal to the following formula:

$F=\frac{1.1R}{L}$

#### kinarfi

##### Well-Known Member
Can anyone guess what it does?
Did you ever try this with a Programmable Unijuction?
Kinarfi

#### Hero999

##### Banned
No but I'm sure it's possible with a programmable unijunction.

You can build an LR oscillator using a comparator or just transistors. There's little point in building an RL oscillator, apart from determining the value of an unknown inductor.

#### kinarfi

##### Well-Known Member
No but I'm sure it's possible with a programmable unijunction.

You can build an LR oscillator using a comparator or just transistors. There's little point in building an RL oscillator, apart from determining the value of an unknown inductor.
I'm sure you're right, but every experiment teaches you something, and of course, so does every failure, if you can figure out what you learned I guess I just gotta see if I can do it.

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