circuit to rewrite the address on the bus, I think it’s not
practical to connect more than one board to a pair of I2C
A Raspberry Pi has several GPIO pins, though. Enough
to handle four boards/eight pins. Since the GPIO pins are
3.3V high, that will work well for the SparkFun board, as
it expects 3.3V high signals. Each board will have to be
controlled separately, changing the pins used to implement
the I2C protocol for the board you want to address.
If that 10 foot distance is not enough, it occurs to
me that it might be interesting to connect the SparkFun
board to a cheap Arduino with something like a Zigbee/
Xbee shield. There are also Zigbee interfaces available for
a Raspberry Pi. It’s much more software, but it would be
possible to collect and send data from each board over
Zigbee to the Pi where it would be collected and analyzed.
It’s not the cheapest solution, but it would be the most
flexible. Zigbee’s range can be up to about 50 feet in a
good RF environment.
QCould you help me with a schematic for an LED candle design that creates a flickering fire effect using only analog components? I’d like to avoid a microcontroller.
Arlington Heights, IL
AThis is a fascinating question. I love doing things in analog circuits that are usually done with microprocessors. I had not thought much about his topic before, and had to do a bit of reading
to see how other people had simulated the effect. It seems
like the key insight is to use two or three oscillators to
simulate the flickering effect. In the end, this might or might
not be easier than using a microcontroller, but let’s see.
The trick is to sum these oscillators to get a resultant
signal to drive an LED. The summed signal will have
epicycles and that might fool your brain. It may take some
tuning of the relative frequencies to get the desired effect,
and three oscillators will probably look better than two.
To make this truly worth it, however, we have to find
an oscillator design that doesn’t use many components.
The simplest transistor oscillator — at least from a
component count standpoint — is a relaxation oscillator
based on a unijunction transistor, or UJT. Something like
the commonly available 2N2646 should work.
A very simple oscillator can be realized with the circuit
in Figure 4. We’ll have to use different time constants to
get the flickering effect.
In Figure 5, I’ve drawn a sample circuit that will
sum the outputs of three oscillators to create an LED
drive waveform. LEDs tend to have a somewhat linear
relationship between current and brightness, so we’ll use
a current source as the driver. The current source doesn’t
have to be perfectly linear, but just enough to get the
QUESTIONS and ANSWERS
i.MX6 ARM CPU
WiFi & Bluetooth
Solo or Quad Core
2 GB DDR3 RAM
Computer on Module
; FIGURE 4. Unijunction transistor relaxation oscillator.
14 January 2018