All About Timing -
Circuits using Resistor and Capacitor Interaction to make them ‘tick’…
Originally published in The
Printed Circuit, Newsletter of the Tallahassee Amateur
Radio Society, January 2013, page 13
For only a
few bucks at Amazon, eBay, AliExpress, and etc. you can get your very
own "solderless" breadboard for building circuits. The term
“Breadboard” comes from the early days of radio and electronics when
experimenters actually built home-brew circuits on cutting boards or
anything wooded they could find lying around the house.
Today’s breadboards are made of plastic and have miniature sockets
which are designed to accept standard integrated circuits and other
small components. The socket holes are spaced 1/10th of an
inch apart and are clustered into logical strips which allow you to
interconnect component leads together. Rigid jumper wires or
Dupont cables (popular with Arduino projects,) which you can get from
the same suppliers are used to complete circuit connections. .
Various sizes of breadboards are
available and can even be interlocked to form super-sized layouts. The
advantages to using a breadboard are many: No soldering, for one, and
you’re allowed to change components and connections at will to suit
your building and experimentation needs. Since socket spacing
is standard, you can often transfer your circuit layouts and
connections to ‘perf’ boards for permanent soldered installation for
your projects. There are a few limitations to breadboards you
must be aware of, for one, they are not designed to accommodate high
voltages and currents, also, allowable component lead thicknesses have
a limit. Breadboard installations are not intended to be
permanent either. Also, any circuit you build must
accommodate or mitigate electrical nuisances such as stray induction,
capacitance and intermittent connections due to oxidation on contact
A breadboard can be typically broken down into
several parts. On the edges of a typical breadboard you’ll
usually find these skinny bars that remind you of a KitKat chocolate
candy piece because they’re usually made to break off separately from
the device. These are the bus strips and usually have two
parallel rows, often in five-hole clusters and the entire single row of
clusters are bonded together lengthwise with one connection strip.
The blue and red lines often printed on the working surface
are an indication of this arrangement. This section is
intended for access to circuit (+) voltage and circuit (-) ground.
On larger boards, there will often be a non-connected gap in
the center which is useful for running multiple power sources for more
complex circuits, but you can add jumpers to form a complete section. .
Breadboards typically have these two bus
bars which are not electrically connected and the main body contains
duplicate clusters of five-pin socket strips and a large recess through
the middle between them. The socket strips are arrayed in
columns (compared to the bus rows) down the length of the board.
Each five-pin cluster electrically separate and can
generally number 30 columns for smaller boards, 64 for larger boards
and on and on. The recess is an "IC Groove” which
is just a physical separator and serves as space to fit an IC removal
tool (or small slotted screwdriver) to remove IC’s (Integrated
Circuits) without damaging them. By the way, you
should consider getting one of those plus a pin straightener if you
would like to build with IC's. IC's that have
dual-inline-pins (DIP) are inserted first, straddling the center groove
with each pin going into the first hole of an associated column of
socket strips. The remaining exposed holes of each associated socket
strip column are then essentially an electrical connection to each pin
of the installed IC. .
Next, you can install any transistors
and 0.01" pin-spaced discrete parts, each pin getting its own socket
strip. The remaining discrete components such as resistors,
capacitors and diodes can be added. When inserting those
parts they can do double duty as jumpers to physically tie in
associated parts of the circuit. Finally, add jumper wires
where needed to complete the circuit and use the shortest lengths that
is feasible. For all the parts that won’t fit on the board,
you can use ‘alligator’ clip wires to add them in. Simply
clamp one clip to a jumper to connect it to the main board.
And remember to double-check your wiring before applying power to your
Let’s get building! .
- A Free-Running Astable Multivibrator Circuit
for toys, models and more! .
Here’s a simple circuit that has a
certain symmetrical quality to it. It’s easy to build and is
often included in STEM education electronics lab kits to demonstrate
basic transistor and resistor/capacitor (R/C) action. You’ll only need
two NPN transistors of any type, two electrolytic capacitors, four
resistors and two LED's (light emitting diodes). LED's are
one of the most important inventions of modern times and can be
purchased in mass quantities for little money from Amazon and
others. [I used a set of their super-bright ultraviolet LED's
is the example shown, why? Because they’re really
cool!] This circuit has many more uses and subtle electronics
design features than is apparent to the new circuit builder; yes, you
can flash LED's which would be good for enhancing toys of model train
sets but there are many electronics applications where simple pulse
signals are needed. This type of circuit is called a
“free-running astable multivibrator,” which means it cycles back and
forth continuously. When included within other circuits with
only slight modification, you can also add basic “flip-flop”
functionality such as dividing-by-two where needed. .
Let’s follow circuit operation.
The transistors are always in opposite states, when Q1 is in
it's 'on' state, Q2 is 'off', and vise-versa. The switching
between states (flip-flop) happens within microseconds because of
regenerative feedback between the two transistors. R1 and R2
serve as base-biasing resistors. If no feedback was provided
for the capacitors both transistors would remain 'on' but in this case,
the job of each capacitor, in its respective side, is to hold
the transistor on the opposite side in an 'off' state through
reverse-bias. The flip-flop action is caused by
interplay between the charging and discharging of the capacitor and
bias switching of each respective transistor.
If C1 is charged it will
hold Q2 in its 'off' state. Q1 is then 'on' and 2 is quickly
charged through Q1's base-emitter junction and its series path of
R4/D2. R2 keeps Q1 'on' since C2 is in its charged state.
During this time, C1 is slow discharging through R1 and the
collector-emitter junction path on Q1. As long as C1's
voltage is high enough a reverse-bias is maintained on Q2, keeping it
'off'. Before C1 is completely discharged the low
collector-emitter voltage on Q1 allows current through R1 to put Q2 it
its 'on' state. Once Q2 is 'on' C2 then turns Q1 to 'off'.
Once Q1 is 'off' each respective LED is subsequently turned
off and on through its associated transistor.
Timing for D1 through Q1 is controlled by values for the R2/C1 pair and
D3 through Q2 by values for the R1/C2 pair. The values given
in this schematic will yield about a 1 second cycle.
Adjusting the bias-resistors R1 and R2 will give you a broad
range of timing but make sure the resistor values are not
below at least 4.7k ohms or you'll damage the transistors! If
you need faster timing reduce the capacitor values. If you
make the timing slow enough, you can use a voltmeter or oscilloscope
with the (-) to ground and the (+) at various locations of the circuit
to correlate the rise and fall of voltages with circuit operation.
digital operation which is able to produce a really clean square-wave
output, because of capacitor discharge, there is a slight gradation to
the dimming of each LED when switching to ‘off’ state. By the way, you
can use flashlight bulbs or relays just the same. You can
replace the LED's with additional transistor switching circuits or a
digital buffer to clean up and provide an output square wave signal.
Check out IMSAI Guy on YouTube give an explanation
including other nuggets of knowledge on the subject: .
. . A 555-Based Event Timer with Relay - with Many Real-World Applications! .
Here’s a circuit with more uses than you can
count. It’s a one-time single event timer (monostable) that uses
the legendary 8-pin “555” timer integrated circuit (IC) and only a
couple of parts. While this one uses an IC, the principals of
using resistor/capacitor elements for timing are the same as with the
above LED flasher. .
Closing S1 momentarily starts a timing cycle. The relay, via NPN
transistor (3904) is actuated during the entire cycle. *C1 and *R1
control the time delay. Since the ‘555’ uses an onboard
comparator, the timing will remain relatively unchanged regardless of
supply voltage. C2 and C3 are used to prevent false
triggering. D1 absorbs reverse voltage (kickback) created by the
relay coil when it is deactivated and with without this diode you may
damage the transistor. .
Pin 3 of the ‘555’ (Output) can handle up to 180ma at 4.5v, but it’s
good practice to use a transistor or MOSFET to take the brunt of
relay and heavy circuit operation. The '3904' transistor is rated
to handle constant current up to 200ma and an addition power transistor
or current-capable MOSFET could be used to switch in larger
current-drawing devices or relays. If you do not have a relay but
wish to experiment with the circuit, simply connect and LED in series
with a 330 to 1k ohm resistor from pin-3 to ground. .
Typical delays can range between milliseconds to several minutes with
this particular circuit configuration. Since components are
affected by temperature the charge and discharge rate of the C1/R1 pair
may not be consistent in this application and timing may vary, but not
appreciably. This type of circuit is to be used for non-critical
timing delay events.
timing rates down to split seconds are useful for devices such as
‘switch debouncers’. Larger rates are great for temporarily
activating devices such as even temporary lighting and cooling fan
over-time control on PC's and ham radio transmitter finals. When
I constructed this circuit I was able to turn my halogen desk lamp
on or off after different durations to demonstrate what this little
circuit could do. I was basically controlling a mains-powered
device (120 volts) from a nine-volt battery powered circuit on a
. This chart shows some timing results I got from varying the values for *R1 and *C1
Use extreme caution and proper safety measures when connecting
line-powered devices to relay connections! Let it be known this
article wasn't intended to pair newbie electronics learners with
mains-voltage projects. ONLY work with mains voltage if trained
to do so! . Have fun building and experimenting with your electronics project this weekend!