Matthias' Blog

In a previous article , I described how I created a people counter that used capacitive sensing. In this article, I will describe the operation of a people counter I made from discrete components that works on a similar principle. As described in the previous article, this circuit will contain two separate oscillators, with each one connected to a sense wire. However, in this circuit, one of the oscillators will be a voltage controlled oscillator (VCO). A frequency comparator will compare the frequencies of the two oscillators and tune the VCO so they match. Schematic and explanation gEDA schematic file Regulator U1 regulates the 12 volt input into the 5 volts needed for the logic gates. The capacitors C1 and C2 are actually placed near the power pins of U2 and U3. Reference oscillator (bottom left box) The reference oscillator oscillates independently of any other signal, except for the capacitance on the sense wire. The circuit is identical to the one described in the prev

In this article, I will describe the need for and operation of a timer circuit when using a mechanical doorbell. This circuit switches the solenoid in the doorbell off after about a second and keeps it off even if the user continues to hold down the doorbell button. By switching off the solenoid, the power dissipation is reduced, preventing the solenoid from melting. Background My doorbell is not powered with an AC transformer, but from a 12 volt battery. The doorbell was recently damaged, most likely by the following prcess: The doorbell button got stuck in the closed position or was held down for too long. Since the solenoid has a resistance of around 6 ohms, the solenoid dissipated around 24 watts, causing it to heat up significantly. The plastic armature softened and compressed, causing the plunger to stick. The plunger heated up and melted the plastic tips on the plunger that would normally strike the bells. The spring that pulls the plunger back started to lose i