Hi - I'd like to have two LEDs (a white and a blue) throb in a sort of
strange way.
Throb?- sounds like a porn thing.
Let's say the white one is at full brightness, I'd like the
blue to be completely dark. The white would slowly start fading, and the
blue would start lighting up. At the point where the blue was fully lit,
the white would be dark.
The simplest way is probably to use a 555 timer- since you say the
alternation frequency is slow, the CMOS LMC555 is best. A circuit for 9V
is shown below:
Please view in a fixed-width font such as Courier.
~
+------+----------+----+----------------+-----------+---|>|---+
| | | | | | Db |
| | +--+----+-----+ | | blue \
| \ | RST VCC | / | 470
| 1M | | 1.5M 4.7k | Rb \
| Rd \ | OUT+-/\/\-+ / | /
| / | | Rt | \ | |
| | | LMC555 | | | c e
| | | | | | |/ |/
| | | | +-------+----+----+---------+
--- | | | | | | | n2|\ p2|\
- 9V | | | | | | | e c
| | +--+TRIG | | | e | | |
| | | | | | | |/ | | +-----+
| +----+--+THRESH | | +-+p1 | | |
| | | | | | | |\ | +---|-----+
| | | | DIS GND | | | c c | |
| \ | +--+----+-----+ | Ct| | |/ | \
| 1M | | | | === +--+n1 | 470
| Rd \ | | | 330k | 1u |\ | Rw \
| / +-------------/\/\--+ FILM e | /
| | | | Rd1 | | | ~ |
+------+----------+----+------------+--------+----------+-|<|-+
Dw
n=2N4401 p=2N4403 white
The alternation period will be 2.5 x Rt x Ct = 3.75 seconds for the
values shown. Increase Rt and Ct for indefinitely longer periods, use no
larger than 10M for Rt and an inexpensive metallized polyester film
capacitor for Ct. p1 and n1 buffer the capacitor voltage onto the
parallel connected bases of n2 and p2 which are complementary emitter
follower drives for the LEDs so that the LED currents have the required
opposite phase. Since you are using blue/white LEDS, the current
limiting resistors, Rb and Rw, are sized for approximately 5mA at a
forward LED voltage of 3V. The THR/TRIG bias network, consisting of 2x
Rd and Rd1, is there to ensure complete LED turn-off and is optional.
The values shown modify the effective 555 switching thresholds to be
approximately 2V of each rail which ensures that each LED is turned off,
or very dim, for practical purposes. In the case of lower voltages, the
bias network may not be necessary so you can remove the 2x Rd and
replace Rd1 with a short to the THR/TRIG inputs.