The circuits in Fig 1 invert the ubiquitous 555 timer's operation. The circuits' output polarities are the inverse of conventional 555 timers'. The circuits' output duty cycle is exactly 50%. Further, you have a choice of open-collector or totem-pole output-or both. And you can feed the timing capacitor from a negative current source referenced to the circuit's common.
I recommend the Texas Instruments TLC555, but any 555 timer should work. The TLC555 has the advantage of being CMOS. Its low power consumption does not introduce switching spikes into the power supply. The TLC555's output also tends to saturate better in the positive direction than devices from other CMOS 555-timer vendors. This feature suits the TI device better for this application.
One disadvantage of the TI device is its limited output-sourcing capability, which limits the capacitor-reset current to 10 mA max. On the other hand, this limitation could be an advantage under some conditions if you use the current-limiting property to eliminate the reset-resistor, R2.
In the circuits, the timing capacitor, C1, ties to the positive bus to keep its discharge current out of the power supply. However, connecting C1 to the negative bus causes the 555 timer to wake up with an opposite-polarity output-a little-known feature of the 555 timer.
The circuits simply bypass the timers' pin 5, the control input. The control input is the top node of the 555's internal voltage divider. In these circuits, bypassing this node to VCC via C2 stabilizes the devices' upper threshold. But the lower threshold is at the mercy of the power-supply bus. Therefore, I recommend bypassing the power supply with a healthy sized capacitor to minimize jitter induced in the 555's lower threshold. This extra capacitor is one disadvantage of these configurations.
The low-voltage operation of the circuits is surprisingly good. The circuits that employ a TCL555 operate easily at 5V. A Schottky diode could further enhance low-voltage operation at the expense of increased diode leakage-a possible problem.
In the sixth circuit, you must use a well-regulated supply because the 555's bandgap reference operates from a voltage divider. Such a setup has no power-supply rejection. (DI #1694)
