Speed controller causes low-speed "cogging"
I have recently completed the ‘Improved Speed Controller For
Universal Motors" as featured in the October 2002 issue. It is driving a brush
motor rated at 2A; well within the design’s 5A rating.
The unit controls OK at the higher speeds. However, when turned
down, it "hunts" from a high speed to virtually off and then accelerates back to
the high speed and so on. I have had one of the ANU’s technicians check the unit
but he cannot identify the problem.
Years ago I built the controller’s lower-tech predecessor and
that controls the motor quite well. Any ideas as to why the newer unit is
causing problems? (A. D., via email).
It is possible that
diode D3 is faulty and is causing disruption with the back-EMF and hence the
speed control. Note, however, that the minimum speed for an appliance is
dependent on the onset of "cogging", where the motor tends to run in short
bursts. So VR2 needs to be set so that the minimum speed is high enough so this
cogging does not happen.
The slowest minimum speed varies from appliance to
Simple mixer circuit wanted
I want to build a preamp which will combine the left and right
signals from my digital set-top box, to enable me to drive a centre speaker
(such as that sold by Dick Smith Electronics). I do not require ultra hifi, as
the idea is simply to improve the clarity of speech. What kit(s) can you
suggest, please? (T. S., Claremont, WA).
The easiest way to
do this is to make up a mixing lead that connects to the left and right outputs
from the digital set-top box and combines the left and right signals using a
10kW resistor in each signal wire. The junction of each resistor then becomes
the centre channel signal for a power amplifier.
A commercial power amplifier can be used or there are many kit
amplifiers available such as the SC480 (Jaycar kit KC-5345) and the accompanying
power supply (KC-5347). The required transformer is the MM-1095 from Jaycar.
Note that the amplifier and power supply must be built into a suitable case.
If you build your own amplifier, the two 10kW mixing resistors
for the left and right channel signals can be placed at the signal input to the
amplifier rather than using a mixing lead as mentioned above.
Software for Smart Card Programmer
I cannot find files for IC-PROG105a on www.ic-prog.com for your Smart Card Programmer (SILICON
CHIP, July 2003). I can only find IC-PROG105e. Is this file sufficient? Can I
also program the PIC877 smartcard with the same software? (T. T., Westmeadows,
The later "e"
version of IC-PROG 105 can be used. For the PIC16F877 you need a loader. The GSM
a3 gold and silver wafer card loader http://users.net.yu/~dejan/ should be able to be used. More information is
available on the net – see http://gsmhosting.com/vbb/archive/index.php/t-37383.html
Speech filter to reduce background noise
I am wondering if you have published a circuit for an audio
filter? I am designing some equipment that needs a filter to remove background
noise and focus on the frequencies of speech. (N. H., via
Strangely enough, we
have not produced an audio "speech" filter although it would be easy enough to
do. Basically, you need a combination of a high-pass filter with a low-pass
filter. To see examples of high-pass and low-pass filters, have a look at the
Subwoofer Controller in the August 2007 issue, specifically at the filter stages
involving IC1b (low-pass) and IC5a (high-pass).
You would need to scale the capacitors or resistors in the
filter networks to provide your speech filter function.