Transistor question

[Dave John]

Please excuse my ignorance, I'm just getting started doing little random projects to introduce myself into building fun/practical circuits.

The first project I'm doing is an LED light table which reacts to hand movement, I plan to achieve this by using LED emitters and receivers to read the light reflected off an object to turn on a coloured LED, most people appear to use a single colour LED but I want to be able to change the colour based on a three switches, one to turn on each of the RGB LED legs.

Each coloured LED in the table will show the same colour but As each IR receiver is linked to it's own coloured LED in a pair isolated from any other IR/LED pair I have to keep the switch almost remotely so it can apply to all LEDs but still maintain their isolation.

I don't want to do PIC programming or use an Arduino so I can keep costs down but also to make is highly scalable/flexible and cheap

Below is circuit I've drawn, I've not shown the IR emitter and receiver part but it's on the negative line going into the LM358, is a transistor a sensible item to use to achieve what I want? If not what would be a good solution?

**broken link removed**


Thanks,
Dave

 

[MikeMl]

Are the tricolor LEDs common anode or common cathode?

 

[Dave John]

Are the tricolor LEDs common anode or common cathode?

Common Cathode

 

[MikeMl]

How much current through each color LED to get the brightness you need?
What is the forward voltage drop across each color LED to get the brightness you need?

The reason I'm asking is because an LM358 can only source ~25mA, so your transistor wiring is not appropriate.

 

[Dave John]

How much current through each color LED to get the brightness you need?
What is the forward voltage drop across each color LED to get the brightness you need?

The reason I'm asking is because an LM358 can only source ~25mA, so your transistor wiring is not appropriate.

I'm don't know the drop, these are the specs I can find:

Forward
Voltage(V) Dominant
wavelength(mm)
MCD Reverse
current(uA) 50% Power
Angle (deg)
　 If=25mA If=25mA If=25mA Vr=5V
　 Min Typ Min Typ Min Typ Max
Red 1.8 3.5 615 625 7000 9000 7 20-25
Bright
Green 3 3.5 515 520 7000 9000 7
Blue 3 3.5 460 465 7000 9000 7

 

[Dave John]

This may be easier to read:
**broken link removed**

 

[MikeMl]

We are getting there.

Now I need to know the characteristics of the signal that comes from the IR detectors. Are you trying to make the LED brightness (sort of) proportional to the IR detector output?

If so, what is the min signal (when no IR is being detected)?

What is the max signal (when a lot of IR is being detected)?

How linear is the relationship between IR signal level and LED brightness? (This one is tough because the eye's brightness response is highly non-linear).

 

[Dave John]

I'd not planned on controlling the brightness specifically, with the circuit shown below I kind of assumed I could use a resistor to drop the voltage output from the OP-Amp down to the 3/3.5v range with a resistor for the LED

This is the IR detection circuit I'd planned to use

**broken link removed**

To quote the source (**broken link removed**)
The 2 resistors R5 and R6 form a voltage divider which provides 2.5V at the anode of the IR LED (here, this led will be used as a sensor). When IR light falls on the LED (D1), the voltage drop increases, the cathode's voltage of D1 may go as low as 1.4V or more, depending on the light

 

[MikeMl]

What a poor excuse for an IR detector. It makes a better thermostat than an IR detector. It needs some hysteresis. The input bias current of an LM358 flows out of the input pin, backwards through the IR led, so the voltage shown as 1.7V in the schematic only exists there while that node is loaded with the resistance of a voltmeter or scope probe.