Microswitch debouce circuit plus high impedance points - case study

Question

I have a circuit where 3.3V are used for MCU and other peripherals, but microswitches are running on 12V. In the first version, microswitch was connected to the base via base resistor of a BJT. What I realized is that the process of switching was producing spikes in the near lines, high enough to trigger my external interrupt of the MCU. I have never seen something like that in practice. So i decided to put rc filter between microswitch and base of BJT. First try has shown that spikes are gone in the near lines. The circuit is shown below.

^{simulate this circuit – Schematic created using CircuitLab}

The idea is to have debounce circuit to prevent input of microcontroller swing for the firs 20ms of the swich vibrations, plus to filter out spikes.

Now, a friend of mine, warns me that 200k ohms in the base circuit is too much, and that one should be aware of such a high impedances in base circuit, cause every induced voltage between Rb and R1 could go to the base, since to the left is 200k. Before I ask my question I wanted to say that I want to have interface between microswitch and MCU via transistor, and I will not consider divider solution.

My questions are: Should I move debouncing between collector and MCU, and use RC filter with smaller time constant in the base in order to remove spikes?

Could you please explain a bit more why high impedances in base circuit are possible problem, as well as all points of high impedance in one circuit.

Whar are possible problems with this circuit?

Cheers

Answer 1

You don't need a transistor to do the de-glitch; just use a potential divider to pot down 12v to something like 3V. A 4:1 divider will work using a 1Mohm and a 330k resistor (attenuation converts 12V to 2.98 volts). Then put a 1 uF (or 2uF) across the 330k.

If there's any possibility that the 12V is higher maybe have a protection diode to Vss from the resistor junction. If you have a regulated 12V then all should be fine.