Hello Michael,

Well, let’s honor Mr. Wizard’s demonstrations and keep his spirit alive in this new era !

I made experiments with a spring wire extending upwards (which I also use as FM antenna) and the needle moves from about 20 cm when approaching L1.

Even rubbing the plastic makes the needle move !

Concerning the question about the Germanium transistors, these are not usable here because they leak a very small amount of current, which is amplified and the output stays high.

Today, we are going to build a Touch dimmer. We will combine two properties of the Darlington pairs.

Let’s build the circuit first with our Lectron start and expansion set.

Darlington dimmer

You can see from the diagram that we use 2 Darlington pairs. The first pair (T1 & T2) form the touch amplifier.

The 2nd pair (T3 & T4) form a sensitive amplifier with very high input resistance which forms (almost) no load on the previous stage.

When you touch the 2 points indicated by the arrow (don’t touch the metal plate !), the lamp will gradually become brighter and the meter needle moves. When you release the touch points at a chosen time, the lamp retain for a very long time it’s brightness.

So, how does the circuit works ?

Darlington pair 1 amplifies the very weak current flowing through your fingers and feed it to C1 & C2 through R1.

C1 & C2 slowly charge since R1 is relatively high.

The charging current flows also through R2 to the 2nd Darlington pair. The lamp and meter connected to it’s output indicate this slowly increasing amplified current.

Now, when you release the 2 touch points, the current charged into C1 & C2 can only flow through R2, since T1 & T2 block.

Now, the magic happens. Since Darlington pair T3 & T4 has an Hfe  of about 32400 and it’s also connected as an emitter follower, it poses (almost) no load on the discharging current of C1 & C2.

This results in a ultra long time till C1 & C2 discharge completely.

Feel free to experiment with different values of C1 & C2.

Also change the values of R1 and/or R2 into 10K and watch the results !




  • This reply was modified 2 years, 8 months ago by Michael.
  • This reply was modified 2 years, 8 months ago by Michael. Reason: Changed font and size styling for easier reading