alan smith wrote:
> So just making sure I understand the ascii art...bottom circuit
>=20
> PNP device, base has a threshold of 1.5V (divider from the +5), emitter i=
s
> the input thru a 4.7K, collector is tied to the base of the NPN. emitter =
of
> the NPN is tied thru a 100K to -5V, and the collector has a pull down to
> ground, and is the output.

No, the *base* of the NPN (Q3) is tied to -5V thru the 100k resistor.
The emitter goes directly to -5V.

And FWIW, I would call the collector resistor a "pull up" to ground.

> 0V on the input keep the circuit off, so the collector of the NPN is low.
> When a 5V input is applied, it bias the base of the NPN, causing it to tu=
rn
> on, providing the -5V output.
>=20
> Do I have that correct?

Pretty much, assuming that when you say "the collector of the NPN is low" y=
ou
mean close to 0V (not -5V). (I would call this "high" relative to -5V.)

You'll want to adjust Vth downward by one Vbe, to about +0.8V, in order to
have the circuit switch when the input crosses +1.5V. You could also use a
silicon diode to establish Vth at ~0.7 V, for a switching level of ~1.4V,
as shown below. Resistor R4 puts ~1mA through the diode.

            Vin o-------+      +---o +5V
                        |      |
                   R1  4k7    4k7  R4
                        |      |
                        |  Q2  |
                        |  PNP |
                        E      |
                         B-----+  Vth =3D 0.7V (for 1.4V switching)
                        C      |
                        |      V  D1
                        |     ---
                        |      |
                        |      +----o Gnd
                        |      |
                        |     470r  R2
                        |      |
                        |      +----o Vout
                        |      |
                        |      C  Q3
                        +-----B   NPN
                        |      E
                  R3  100k     |
                        |      |
                        +------+----o -5V

-- Dave Tweed
--=20
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