555 astable with LED at R1

[socratidion]

I have a standard 555 astable mode circuit -- I'll refer to the circuit diagram in the Wikipedia article, https://en.wikipedia.org/wiki/555_timer_IC

It's powered by a 9V battery, and the output is going through a small resistor straight to a speaker.
R2 is 10K
C is 10nF
Instead of R1 I have a choice of two resistors: one is 167K, which gives me the pitch G5. The other is 191K, which gives me the pitch F5. I have momentary switches, which I can place either immediately before or after each resistor to get one of those two pitches.

I thought it would be a cute idea to configure it so that when I pressed the 'G5' switch, a red LED would light, and when I pressed the 'F5' switch, a yellow LED would light. What is the simplest way of achieving this, without affecting the pitch of the oscillator?

[That's the question. Here's where I am with it. It seems obvious that putting an LED into the path won't work, because the resistances are too high, and the LED won't have enough current. The absolute simplest solution would be to use DPST switches, so that the LED circuit is completely separate. But I want to avoid that because I'm very fussy about the design of the buttons, and I haven't found a DPST version of the switches I like, so I'm working with SPDT (or SPST).

So the first thing I tried was using the little current there was when the switch was closed to trigger an NPN transistor, i.e. branching off and sending current to the base. It worked, but the pitch of the oscillator dipped by about three semitones.

The second thing I tried was with a SPDT: the common terminal connected to pin 7 of the 555, then the other terminals connected (1) to the resistor path, and thence to the positive rail, and (2) to the gate of an n-channel JFET which controlled the current for the LED. The idea was that when the switch was in the second position, and the chip was not oscillating, pin 7 would be grounded, and the JFET would have enough negative voltage at its gate to block current and switch off the LED. Results were discouraging. The oscillator pitch was unaffected by the LED, but I could barely get the LED to switch on (dimly), or indeed switch off once it had switched on. I expect it has something to do with the voltage I've got at the source, but all this stuff is very new to me, and I think I just need some advice.

Maybe ultimately I will just change my mind about DPST switches, but I'm bothered that I haven't been able to solve what ought to be a very simple problem…]

 

[GUMY]

You should do it in opposite way. Control LED's directly with mechanical switch ad use small MOSFET to switch resistors.

 

[socratidion]

You should do it in opposite way. Control LED's directly with mechanical switch ad use small MOSFET to switch resistors.

Thanks for the reply. If I understand you correctly, I would have a separate LED + resistor pathway, with a branch-off to the gate of a MOSFET. This should be a normally off MOSFET (so 'enhancement' type), which I could switch on with either positive or negative voltage depending whether n-channel or p-channel, and depending on where I branch off from the LED path.

I hadn't thought of that: I'll give it a go. Can I rely on infinite resistance -- absolutely no current getting through -- when the MOSFET is off? I've noticed (from trying to make my own switches, inadequately) that the oscillator is extraordinarily sensitive to even a minute amount of current coming from the switch, which produces an irritating low-frequency stutter (just a few Hz).

 

[GUMY]

You understand correctly.
If You disconnect both resistors, You get erratic operation as You allready noticed. You should stop the timer, by pulling reset pin (4) low. You can make simple logic circuit to do that, if no button is pressed.