■ WITH RUSSELL KINCAID
In this column, I answer questions about all
aspects of electronics, including computer
hardware, software, circuits, electronic theory,
troubleshooting, and anything else of interest to
the hobbyist. Feel free to participate with your
questions, comments, or suggestions. Send all
questions and comments to: Q&A@nutsvolts.com
Join us as we delve into the
basics of electronics as applied
to every day problems, like:
Simple Magnetizer ✓
● Random Traffic Lights
QYou were asked to provide a circuit to blink LEDs in the February issue; something equivalent to the
classic blinking neon NE- 2 type light
“relaxation oscillator,” I gathered.
There is another way to do this with
LEDs that I have been using for years,
using a unijunction transistor. It uses
one cap, two resistors, one LED, and
one UJT, and runs on 5-10 volts.
Slightly more complicated than the
old NE- 2 version but runs on low
voltage. You ought to do a story on
— Bob Waber
AI guess you are right, Bob. I thought the unijunction was obsolete. Someone gave me 5,000 of them
once, but I finally threw them away.
This device is 40 or 50 years old, but
■ FIGURE 1.
I see that they are still on the
market, so it is worthwhile
explaining how they work.
The 2N2646 and 2N4891 are
the old style (non-programmable)
UJT and are not available anywhere
— except from NTE at an exorbitant
price. The 2N6028 and 2N6027 are
programmable unijunctions which
have a different schematic but work
the same. The 2N6028 and 2N6027
are the same device, but the
2N6028 has tighter specs. The
programmable unijunction is a four-layer device, similar to an SCR (see
Figure 1). The N layer is biased by
RB1 and RB2, and sets the trigger
When the top P-N junction is
forward-biased, current flows
causing an avalanche (negative
resistance effect). The anode is
pulled down but can’t go as low as
the old style UJT because of all the
junctions. The 2N2646 could go
down to a diode drop of ground; the
2N6028 goes only to 1.5 volts. This
avalanche effect is a problem for ICs
■ FIGURE 2.
which have many junctions. If the
wrong PN junction is forward-biased,
the IC will be destroyed.
The relaxation oscillator
(Figure 2) is easy to design but
there are two considerations:
1. R1 must provide enough
current to start the avalanche.
In the datasheet, this is called
peak current (Ip).
2. R1 must not provide more
than the holding current, or
the UJT will not shut off and
start a new cycle. On the
datasheet, this is called valley
To find the maximum value for
R1, the peak voltage (Vp) is found
by: Vp = Vd + Vcc*Rb1/(Rb1+Rb2);
then: R1max = (Vcc-Vp)/Ip.
Note: Vd is the P-N diode drop
at Ip; about 0.4V.
To find the minimum value for
R1, look up the maximum forward-voltage on the datasheet (about 1.5
volts) and the valley current, Iv; then:
R1min = (Vcc-Vf)/Iv. Note that the
valley current varies with R1, so this
is only an approximation.
In Figure 2, D1 is an LED that
flashes each time the capacitor
discharges. Computing the
frequency is difficult because the
capacitor charging is exponential
and depends on where the trigger
voltage is relative to Vcc. If you set
the gate voltage at 0.63*(Vcc-Vd-
Vmin), the frequency will be: F =