digital-communicationsswitchessynchronization
The following diagram is a practical diagram for derivation of the clock signal from the received waveform itself (Synchronization) in baseband PAM systems.
I'm looking for a typical circuit for the "Transistor current switches". I have no Idea of how it should be designed.
To return to your transistor solution (as others have opted for a different mechanical switch), it might look like this. I included Wouter's hint for using a diode instead of the second transistor. Also I changed the bipolar transistor to a MOSFET. Reason for a MOSFET is because the base current of a bipolar transistor gets rather high with regular power transistors. Reason for P-channel high side switching is that this way the load can be attached to ground. If this is not a requirement, the circuit can be slightly changed to support an N-channel MOSFET.
simulate this circuit – Schematic created using CircuitLab
Your original schematic is somewhat confusing, as it does make it look like you are trying to short-circuit the USB power rail. I have redrawn it below with a circuit that should do what you want. It uses the switch to pull down the gate of a P-channel enhancement mode MOSFET.
simulate this circuit – Schematic created using CircuitLab
The main point to note is that the current to drive the LED no longer comes from the switch IC, but instead directly from the USB power rail. The switch input is not intended to provide significant power, and to get the current amplification benefit from the transistor it needs to get its current from a higher amperage source.
You could achieve this using a PNP BJT, but that will have a fixed saturation voltage across the collector and emitter of the PNP transistor (around 0.15V) requiring a recalculation of R1. It also requires a series base resistor whose value must be calculated with the Hfe of the BJT in mind. It does have the advantage of not placing a low gate threshold voltage requirement on the transistor, and more suitable PNP BJTs are available in through-hole packages than P-channel MOSFETs.
This circuit uses the BC557 purely because it is the complementary part to the BC547 you used in your original attempt. The guaranteed Hfe of the lowest grade (A) parts is 110, so for -20mA collector-emitter current we need at least 180uA of base current. Using a 4k7 base resistor we have 5-0.7 = 4.3V across it, giving a base current of 4.3/4.7k = 915uA. This is enough to drive the BJT hard into saturation without excessive base current flowing. Note that with the MOSFET version of this circuit there would be practically no gate current flowing, so you would gain greater efficiency that way. I haven't changed R1 to allow for the collector emitter saturation voltage - the required reduction would likely be around 10 ohms, so you may choose to neglect this.
Best Answer
The circuit labelled "transistor current switches" is called Charge Pump. Here is a "textbook" (ideal) implementation:
simulate this circuit – Schematic created using CircuitLab
M2 and M1 are the "switches". M3 and M4 establish the current being injected into or pulled out of the output. M5 and M6 are for the current reference, which establishes the gate voltages for M3 and M4 for the desired current (set by the current source).
The way it works is that when M2 is on, charge is injected to the output capacitor increasing the voltage linearly with time. When M1 is on (and M2 is off) the output capacitor is discharged at the same speed.
Depending on the requirements of the system, you might be able to get away by simply replacing M3 and M4 with resistors, but the current will be dependent on the voltage held at the capacitor.