Nuts and Volts - July/August 2018

The operation of this circuit can easily be obtained using a 555 replacement in either mode 4 or mode 5. Schematic 5 shows the wiring of the 555 replacement for this equivalent circuit (the resistor values are for mode 5). Note that the PIC uses a transistor buffer to drive the relay.

The PIC output current is limited to 20 mA, so a buffer is going to be required for most relays. Select mode 4 if you want to control the off-time of the relay and its on-time. Select mode 5 if you want to control the period of the relay operation and its on-time. This is a subtle difference but may be important depending on the application.

Also, note that the 555 circuit uses +12V while the PIC should use no more than +5V. A higher voltage relay can be used if desired simply by connecting the “top” side of the relay and cathode of the diode to the required voltage.

Mims Circuits 12, 22, and 23

The circuits in Schematic 6 use the 555 as a simple pulse generator. For FM- 22 and FM- 23, the only differences are the loads the 555 is driving and the timing components.

Its astable operation has been discussed in previous articles of this series, so I won’t repeat it here. The frequency range of FM- 12 is roughly 1.4 Hz to 300 kHz depending on the capacitor. I have included a spreadsheet with the article downloads (555 Astable.ods) which calculates the time values based on the three timing components. Keep in mind that as R1 becomes small, the circuit will stop operating correctly (if R7 = 0).

I found that with C1 = .01 µF or .1 µF, I needed to make R7 about 390 ohms for the circuit to work properly with R1 = 0 and VCC = 5V. Each capacitor value yields about a 400:1 frequency span.

With this circuit, the output is a very narrow low level pulse. For any given range determined by the capacitor, the output pulse width will be fixed unless you vary the value of R2. The following formulas apply: LowPulse Width = ln( 2) x R2 x C1 = .693 x R2 x C1

Period = ln( 2) x (R1 + R7 + ( 2 x R2)) x C1

Due to component tolerances — especially the capacitor — you can use either 0.66 (2/3) or 0.7 instead of the constant shown in the equations, which will get you close enough for the vast majority of applications.

The values shown in the schematic are specifically for FM- 12, the pulse generator. For FM- 22 (an LED flasher), R1 is 100K and C1 is 47 µF. The flash rate can be varied between 0.2 Hz and 8. 3 Hz. The 2N2222 inverts the signal and causes the LED to flash on for a short time (when pin 5 is low) of approximately 32 ms.

Since the 555 has plenty of drive capability that can easily handle an LED directly, it’s possible to get the same results without a transistor using the alternate LED circuit shown in the box. The value of R6 (as well as R4) should be calculated based on the supply voltage and the current required by the LED. The values shown for R6 cause about 5 mA through LED2 at the two voltage extremes.

Just as a note of interest, the original circuit with the transistor draws roughly the same amount of current independent of the state of the LED. The alternative circuit (without the transistor) draws very little current when the LED is off.