siliconchip.com.au Australia’s electronics magazine July 2019 107
reported that 64727 bytes were saved.
I entered the RUN command and the
terminal said:
[685] VAR RESTORE
Error: Variable name
The bottom of the LCD displayed:
Main Standby Config Dig Out
Nothing else was visible. Touching
these areas produces a beep. Is this
normal with the radio board not in-
stalled? (B. K., Iowa, USA)
- It sounds like the BASIC program
has been corrupted during the upload
process and contains an error. The ac-
tual “DAB FM AM Radio Firmware
CRUNCHED.bas” file is 66104 bytes
(not 64727). Try uploading it again,
or alternatively try flashing it with
the HEX file using a PIC programmer.
Larger flip-dot display
wanted
I’ve just built one of four flip-dot dis-
plays that I intend to use for a clock,
as described in your April 2019 issue
(siliconchip.com.au/Article/11520).
It’s being driven by an ESP8266 mod-
ule using internet time (NTP). I imag-
ine others are doing this as it’s a great
application for flip-dot displays.
The display works well, switching
reliably even when horizontal. How-
ever, it is a little hard to read. While I
know that it would require more than
twice the parts, is there a plan to de-
scribe a larger version; say 7x5 pixels?
(D. S., East Melbourne, Vic)- We’re glad to hear that you’re using
our flip-dot display design. Although
we did not go into much detail, the ar-
ticle notes that larger displays can be
created by stacking multiple display
modules, so we don’t currently have
plans to work on a larger version.
If you built 16 modules and con-
nected them eight wide and two high,
that would give you a four-character
display with 6 x 10 pixels each.
Alternatively, since you are display-
ing time and therefore only need num-
bers, you could tweak the font to give
maximum clarity in this role. A font
similar to a 7-segment display might
be more legible for a clock than the
one we provided.
Can a DFPlayer Mini be
added to DAB+ Radio?
I am thinking of building the DAB+/
FM/AM Tuner (January-March 2019;
siliconchip.com.au/Series/330) in the
near future and have a couple of ques-
tions about the design.
Would it be possible to connect a
DFPlayer Mini module, as described in
the December 2018 issue (siliconchip.com.au/Article/11341) to the radio
board via the CON8/CON9 expansion
headers? All the required connections
such as 5V, GND, TX and RX have been
provided. Presumably, this could then
feed audio into pins 4 and 11 of IC6
via pins 6 and 7 of CON8.
Would the existing line output net-
works on the DFPlayer module, as
shown in Fig 2 of the aforementioned
article, be appropriate with possible
value changes? Obviously, a small sub-
board would be needed to match the
pins of the module to CON8/CON9. (J.
C., Creewah, NSW)- What you are suggesting is along the
lines of what we had in mind when we
added CON8 & CON9 to the radio de-
sign. However, we haven’t done any
real design work for an add-on module
yet. Your idea is workable.
The expansion header provides a
way to feed audio into the analog mul-
tiplexer. As long as you can arrange for
the signal level and DC biasing to be
appropriate, any stereo analog signal
source can be used.
You would need to add resistive di-
viders to reduce the audio levels from
the DFPlayer module to those expected
by the radio board; the radio chip it-
self produces around 70mV RMS, and
you would want to match that to avoid
huge jumps in volume when switch-
I have a couple of low-cost but
excellent desktop audio amplifiers
with inbuilt DACs (SMSL Q5 pro).
I use these for video and audio ed-
iting on my PC.
These amplifiers, like most other
similar units, have no provision for
headphones and no line out sock-
ets. This omission restricts their
usefulness at night time and other
odd hours.
Adding headphone outputs is
difficult because these, like many
newer amplifiers, actively drive the
negative speaker terminals; they
are no longer tied to ground like on
older amps.
I cannot use a resistive divider
to connect headphones to the out-
puts as the standard 6.5mm and
3.5mm stereo plugs all use a 3-wire
scheme with a common ground for
both channels.
I have scoured the internet forsimple solutions without finding
anything suitable. Can you suggest
a practical passive or active solution
to this? Would it be a good idea for a
future project? (D. S., Nowra, NSW)- It is possible to drive headphones
from an amplifier operating in bridge
mode, but you need to open up the
headphones, separate the ground
wires to the left and right sides and
wire them to a four-core cable with
a four-pin connector on the end (eg,
mini-DIN).
We have done this before, and it
works fine, but it’s best if you mod-
ify headphones that can easily be
opened up (ie, held together with
screws rather than clips/glue). You
can then make an adaptor cable so
the headphones can still be used
with a regular 3-pin socket, where
the left and right grounds merge in
the adaptor.
Without using transformers
How to connect headphones to a bridge-mode amplifier
(which probably would negatively
affect sound quality), the other op-
tion is to open up the amplifier and
find a ground point on the PCB,
then wire the headphone ground to
this via a high-value electrolytic ca-
pacitor, with its positive terminal to
headphones.
You can then connect the head-
phone left and right signals to one
of the output terminals on each side.
How well this works depends on
the amplifier design, but it should
work in most cases.
Ultimately, we think it’s easier and
better to use a separate amplifier in-
tended for use with headphones. For
example, our High-performance Ste-
reo Headphone Amplifier described
in the September & October 2011 is-
sues (siliconchip.com.au/Series/32).
It sounds great, and it can even drive
speakers, as long as you don’t need
a huge amount of power.