Multi-shot (Multiple monostables on one timer)

This circuit provides a pulse-sequencing capability using the control voltage inputs of the 555.

The circuit described here triggers three (or an arbitrary number) monostables with differing but related widths. What’s different about this one than others I’ve seen is that this circuit has only one timing signal; in particular, there is only one RC network among all, which could potentially reduce the parts count and, more importantly, increase consistency among the timers.

The timing network is actually sort of a modified RC using a transistor and a voltage reference to charge the capacitor with a constant current. The timing is determined by the base voltage of the PNP transistor, the resistance at the emitter resistance, and the capacitance at the collector. The voltage drop across the resistor is the Vcc – Vb + Veb[1]. As long as none of these three values vary significantly in the application, the resistor’s voltage drop can be considered constant. If the resistor is, as expected, also constant, then the current through it is constant as well: I = V/R. The transistor will adjust as necessary, if possible, to maintain that current through the transistor[2]. If the supply voltage is well regulated, the base voltage can be set with a voltage divider.

The output of this constant current timing network, in contrast to the tricky curve of a regular RC network, is a linear ramp. This makes it easier to calculate the desired control voltage for each monostable. In the example above, the control voltages are 1V, 2V, and 3V, where the 1V and 2V timers produce a pulse exactly 1/3 and 2/3, respectively, the width of the 3V timer pulse. The values need not be this regular. A normal RC network could be substituted if you’re willing to do the math.

The monostable with the longest delay should have its discharge pin connected with its threshold pin in order to discharge the capacitor.

This approach gets a little unfortunate where setting the control voltages comes in. The CV is preset internally with a three-step resistor divider network. The CV itself is roughly 5k from Vcc and roughly 10k from ground. If I’ve done the math correctly, this is equivalent to a 2Vcc/3 supply through a 3.3k resistance. This means that you can’t trivially set the control voltage with a divider—something with a low (ideally zero) output resistance is necessary. Here are some things that might be worth trying:

  • Resistor divider with proper buffer (2 resistors, 1/4 to 1 op amp IC). If you’re not willing to complicate the calculations, an op amp can output the same voltage as the input, with no base-emitter drop. As a bonus, it’s an op amp, which is capable of a whole lot more. If restricted to Radio Shack, I’d pick up two TL082 JFET dual amps (or an LM324 quad if they’re out), then use three of the amps with a resistor ladder to set CVs and the one left over to upgrade the current source. I’d guess this is the option that would involve the least hair-pulling. (Edit: An example of a buffered resistor divider, complete with redone current source, in terms of a 7805 voltage regulator. Would require a supply voltage above 7 volts or so, but it’s an easy out in 2-3 ICs and about 10 resistors, plus the transistor and RC already in the circuit.)
  • Resistor divider with unity gain transistor (2 resistors, 1 NPN transistor). Similarly to the current source, the emitter voltage is a nearly fixed drop below the base voltage.
  • Trimmer (1 potentiometer, potentially up to 2 biasing resistors) if you’re okay with a bit of tuning.
  • Linear voltage regulators/references. Would probably work really well, but might be expensive depending on where you buy[3] and in this case requires a fairly small one (probably around 1V). The LM317 can be treated as a precision 1.25V drop, as long as the minimum load requirement is met.
  • Diode drops (1 diode). In the simulator, I was able to get the circuit to work with a ladder of diode drops for the control voltages (i.e., around 0.7, 1.4, and 2.1V). I’ve read that in practice this doesn’t work nearly as well.

By the way, the intended purpose is to trigger some D flip-flops to read serial data in an experimental line encoding I’ve conceived. It’s a variation of one I discussed way back, but the initial rising edge is followed by three bits of data before the mandatory fall. If it works, it could mean a simple output interface for PCs (e.g. via monitor or sound card) to devices that only understand data in terms of data, clock, and latch. What does this mean? You guessed it—semi-headless 74HC595 displays. (Okay, maybe not so much, but that’d at least be a good way to test it.)

This circuit is available on the Falstad simulator.

  1. [1] Veb may vary slightly depending on current, but is roughly 0.7V under normal circumstances.
  2. [2] Up to near the max collector voltage, in this case slightly lower than the emitter voltage.
  3. [3] LM317 is $3 at Radio Shack, but way cheaper online, especially in a TO-92 package.

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