|
Switchable output crystal oscillator
This oscillator circuit permits crystals to be electronically
switched by logic commands. The circuit is best understood by initially ignoring
all crystals. Furthermore, assume that all diodes are shorts and their
associated 1kΩ resistors open.
The two 1kΩ resistors at the non-inverting input of IC1 (LT1016) set the
output to half the supply, ie, +2.5V. The RC network from the output to pin 3
sets up phase-shifted feedback and the circuit looks like a wide-band unity gain
follower at DC.
When crystal X1 is inserted (remember, D1 is temporarily
shorted) positive feedback occurs and oscillation commences at the crystal’s
resonant frequency.
If D1 and its 1kΩ resistor are then considered to be part of the circuit,
oscillation can only continue if logic input A is biased high. Similarly, the
circuit can only operate at crystal X2’s frequency if logic input B is high.
Additional crystal/diode/1kΩ resistor branches permit logic selection of the crystal
frequency.
For AT cut crystals about a millisecond is required for the
circuit output to stabilise due to the high Q factors involved. Crystal
frequencies can be as high as 16MHz before propagation delays in the comparator
prevent reliable operation.
Linear Technology application note.
|
Low Ohms Adaptor for DMMs based on an LM317 regulator
The circuit of the Low Ohms Adapter gives good results for such a simple circuit.
This adaptor circuit is essentially a 100mA constant current
source. It is applied across a low-value resistor of unknown value (ie, the
resistance to be measured) and the resulting voltage drop can then be measured
by a digital multimeter (DMM).
Setting your DMM to the 200mV range will enable it to measure
up to 2Ω with
high resolution while the 2V range will give a maximum resistance measurement of
20Ω.
Construction could consist of mounting the LM317 adjustable
3-terminal regulator inside a small plastic box together with the battery and
two resistors connected to the output and Adj pins.
No on/off switch is required since no current will be drawn
when no external resistance is connected across the Test terminals.
Accuracy using 1% resistors should be within 5% and this could
be improved by measuring the current, adjusting the resistance between the
output and Adj pins of the LM317 to provide a precise 100mA.
Before using the adaptor, check that your meter is not likely
to be damaged by having the full output (6V+) applied when it is set to a low
voltage range.
Similarly, be aware that the voltage and current output of the
adaptor may damage components if you use it for "in-circuit" tests.
Peter Chamberlain, Strathfield South, NSW. ($30)
|
LED number display uses parallel port
This circuit was developed as part of an MP3 player based on an
old computer. The 2-digit display indicates which song is playing. The LED
display has the advantage that it is readable from a distance compared to a
2-line LCD. The circuit could be used for other applications such as a
programmable calculator, stop-watch clock or scoreboard.
The circuit uses a 74HC138 one-of-eight decoder connected to
three data lines from the parallel port and a 4511 BCD to 7-segment decoder
driver for each LED digit.
The software was written using Q basic but should be easily
converted to other languages. This example will output a 4-digit (or less)
number to the parallel port to be displayed on a 4-digit LED display:
| CONST PPort = &H378 |
'default address of parallel port (1) |
| Number = 1234 |
'number to display |
| n1$ = STR$(Number) |
'convert number to string |
| n2$ = RIGHT$(n1$, LEN(n1$) - 1) |
'remove leading blank space |
| FOR a = 4 TO 1 STEP -1 |
'this example uses 4 digit display |
| IF a > LEN(n2$) THEN |
'is this digit used? |
| lo = 15 |
'not used, blank it out (can be 0) |
| ELSE |
'digit to be displayed normally |
| n$ = MID$(n2$, LEN(n2$) - a + 1, 1) |
'get 1 digit only |
| lo = VAL(n$) ‘ |
'convert to byte |
| END IF |
| hi = a * 16 |
'select digit to be displayed |
| OUT PPort, hi + lo |
'write value lo to digit hi |
| OUT PPort, lo |
'select digit 0 (unused) fix timing error |
| OUT PPort + 2, 1 |
'lock display |
| NEXT |
'get next |
Philip Chugg, Rocherlea, Tas.
|
