Questions on a kit schematic

[krich]

I've found a kit from this website that meets my needs for a project I am working on: https://www.kitsrus.com/pdf/k130.pdf

I am fairly new to electronics, however not new to technology (I'm a software guy). I'm learning as I go, so I wanted to understand the schematics of the kit before I build it, rather than just build it blindly.

I've been able to understand most of what's going on in the schematics (5V power supply with the Zener, potential dividers in front of the transistors), but there are a few things that are eluding me. Primarily I am confused as to what the function of the 1N4148 signal diodes (D1, D2) are in the receiver module. Is D2 just there for the voltage drop? What purpose does D1 serve, being that it's simply connected between a voltage source and ground?

Thanks for any input on this.

 

[kchriste]

That part of the circuit is a voltage doubler which is explained **broken link removed** in greater detail. It converts the AC waveform from Q1 to DC to light the LED (L1) and turn on Q2.

 

[Pommie]

I recognise that circuit as a voltage doubler but thought it would only work with true AC and not pulsed DC. In a voltage doubler C1 would charge via D1 on the negative cycle whereas in that circuit C1 discharges via D1 and R3 on the off cycle. Or am I missing something?

Mike.

 

[ericgibbs]

I've been able to understand most of what's going on in the schematics (5V power supply with the Zener, potential dividers in front of the transistors), but there are a few things that are eluding me. Primarily I am confused as to what the function of the 1N4148 signal diodes (D1, D2) are in the receiver module. Is D2 just there for the voltage drop? What purpose does D1 serve, being that it's simply connected between a voltage source and ground?
Thanks for any input on this.

hi,
The square wave pulses coming from Q1 collector are 'ac' coupled via C1 to the rectification circuit.
D1 clamps any 'negative' excursion of the square wave to close to 0V [about -0.7V].
D2 conducts on the positive excursion of the square wave, charging capacitor C2 positive.

When the postive voltage on C2 exceeds the forward voltage drop of L1 LED, the led will light. The current flowing the L1, R4 and R5 will cause a voltage drop across R5, when this voltage exceeds +0.6V the transistor Q2 will conduct and relay RL1 will be operated.

Further filtering of the rectified wave is provided by C3 on the base of Q2, this should stop the relay 'chattering' on/off.

 

[krich]

Thanks guys. That gives me a good direction towards which to barge ahead.

I've already taken a look at coupling capacitors and how they behave and I can see how the "ac" (not really, but close enough for the coupling capacitor) coming from Q1 would be passed on to the rest of the circuit, but a constant low or high voltage would not. That's a very interesting effect. I've seen mention of this effect associated with those soundcard oscilloscope projects, but never understood what they were talking about.

Now on to voltage multipliers. I'm somewhat familiar with the schematics of half/full wave rectifiers, so I'm hoping that's similar in concept.

Thanks again!

 

[crutschow]

I've already taken a look at coupling capacitors and how they behave and I can see how the "ac" (not really, but close enough for the coupling capacitor) coming from Q1 would be passed on to the rest of the circuit, but a constant low or high voltage would not.

The common meaning of "AC" refers to power line current, but, in general, AC (alternating current) is any current that changes in value with time (i.e. not DC). Thus a square-wave is AC just as is the sinewave from the power line and an audio signal from a microphone. The only difference is the frequency content of the signal. And any AC signal can be coupled through a capacitor but DC is blocked.