Re: Sensor/LED circuit-more help needed please-multiV's and dimmer

On Sat, 20 Aug 2005 17:14:06 -0500, John Fields
<jfields@xxxxxxxxxxxxxxxxxxxxx> wrote:


>+VIN>-----+-------+---------------------------------+
> | |A |
> [10K] [LED] |
> | | |
> | C Q1 |
> +-----B 2N4401 |
> | E |
> | | |
> Q2 C | |
> 2N4401 B-----+--[R2]--+ |
> E | | O
> | [R3] Q3 C |
> | | 2N4401 B--[4k7]--+--> >--O--> |
> | | E | S1
> | | | [100K]
> | | | |
>GND-------+-------+--------+-----------+
>
>Vin is the output voltage from your sensor, and Q1 and Q2 form a
>constant current source where R3 determines the current through the
>LED. I just built one to see how well it would work, and with 39
>ohms for R3 I got 17.07mA through the LED with Vin = 14V, and
>17.83mA with Vin = 28V. Not bad!
>
>R2 and Q3 do the dimming, and when Q3 is turned on by closing S1,
>(the common BRIGHT/DIM switch) R2 will be placed in parallel with
>R3, requiring more current to flow in order to bring the circuit
>into regulation. That means that "BRIGHT" will be when the switch
>is closed, and "DIM" will be when it's open.
>
>After I tried the basic circuit I added on the dimmer, and with R2
>and R3 both equal to 39 0hms, here's what I got:
>
> Vin Idim Ibrt
> V mA mA
> 14 16.4 29.8
> 28 16.0 32.9
>
>
>You'll have to fiddle with R2 and R3 to get the ratio of bright to
>dim to be what you want but, basically, it works.
>
>Also, I forgot to show it on the schematic, but I bypassed the
>supply, at the circuit, with 0.1µF.

---
P.S.

2N4401's are rated for 625mW max, and Q1 will be dissipating about
500mW with a 28V supply, so if you decide to use this circuit you
may want to get a transistor that can handle a little more power or
put a dropping resistor in series with the LED or even glue all the
Q1's to a heat sink...

And, BTW, the BRIGHT/DIM switch needs to be wired to the 14V/28V
POWER switch, not to the output of a sensor, as I showed before.

Like this:

+VIN>-----+-------+
| |A
[10K] [LED]
| |
| C Q1
+-----B 2N4401 14V O--> |<--O 28V
| E |
| | O
Q2 C | |
2N4401 B-----+--[R2]--+ |
E | | O
| [R3] Q3 C |
| | 2N4401 B--[4k7]--+--> >--O--> |
| | E | S1
| | | [100K]
| | | |
GND-------+-------+--------+-----------+


--
John Fields
Professional Circuit Designer
.

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