Pretty new to electrical engineering world, so probably rather simple question. I´m designing control system for tractor trailer hydraulic system. The system is 12 V and solenoids what I am using there are 2 A. Also I´m using few digital inductive sensors(IFS241) which output also 12 V. Since There are some logic behind the operations, I was thinking to use 18 V AND CMOS logic gate (NTE4081B) and 20 V NOT CMOS logic gate (CD4069UB) to run relays. But here is the problem that logic gate outputs only 500 mW and already one relay needs 400 mW (DS1E-M-DC12V). Do you have some ideas how to boost this signal, or how I should control the relays with logic?

Thank you for you advice.

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    \$\begingroup\$ Circuitry can always be added to logic output in order to drive solenoids and relays. But more details about the system details you are planning would save everyone time chasing wrong directions. Especially when discussing what must be a safety-related system. I don't know what other details you should add. But I can say that more is indicated. \$\endgroup\$
    – jonk
    Commented Oct 27, 2021 at 7:22
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    \$\begingroup\$ Not to foil the fun but I think a programmable relay would be a better choice for the application (these are like mini-PLC). You don't want to cope to the evils of automotive power supplies \$\endgroup\$ Commented Oct 27, 2021 at 7:29
  • \$\begingroup\$ Have you done any research yourself how to drive relays with a logic gate? Why any of those solutions won't work for you? \$\endgroup\$
    – Justme
    Commented Oct 27, 2021 at 10:11
  • \$\begingroup\$ Thank you for your comments. @Justme I eventually landed on this article ( elprocus.com/relay-driver-circuit-using-uln2003 ) which describes how to use ULN2003 NPN relay driver. But I have one self holding relay in my logic so I was wondering why there isn't any positive side relay drivers. Probably actually it can be worked around. As I said I´m trying to understand what barrier can lay ahead together with the help of this forum . Also, as I understand from Lorenzo Marcantonio automotive power supply isn't stable enough and can cause problems, so some circuitry is needed. \$\endgroup\$ Commented Oct 27, 2021 at 11:27

2 Answers 2


To replace some simple logic such as CD4069UB with higher drive, you can use transistors. Such as MOSFETs or BJTs. Even the most basic transistors can give you power of 400 mW or more. This way, you can control the amount of output power you can drive by choosing a transistor (output current limited by open channel of transistor). If you drive relay when the output of inverter is high, you should take P channel inverter ( PMOS inverter on the picture). If you drive them low, you can use N side inverter. If however you need to have low output impedance on both LOW and HIGH output, use CMOS logic (P and N transistor). Here are all representations of inverter with BJT or MOSFETs. To drive the output of an AND gate, you can invert its output and then use the output inverter as before as a buffer. Also, don't forget to add diodes if you want to control an inductance load. More info here. All inverters


Provided that the logic is simple, the task may be easily accomplished using relays alone.

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    \$\begingroup\$ Especially if the only functions really needed are AND (series contacts) and NOT (normally closed contact). \$\endgroup\$
    – Theodore
    Commented Oct 27, 2021 at 13:51
  • \$\begingroup\$ Yes, should the functions required be AND, OR, NAND and NOR. \$\endgroup\$
    – vu2nan
    Commented Oct 27, 2021 at 13:56

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